Across: 1 Masters course (AUGUSTa) minus the last letter (mostly) = AUGUST ((just over 4 weeks in summer), 4 Hard (TOUGH) with (H)eroin inserted (injecting) = THOUGH (albeit), 8 Mahatma (SAGE) containing (being ingested) 1st letter (leads to) of words 6-8 = SALVAGE (recovery), 9 Double def of TRY IT ON , 11 A + politician (TORY) preceded by (beginning with) injured (GORED) reversed (back) = DEROGATORY (disparaging), 12 Round (O) + with in German (MIT) = OMIT (fail to mention), 13 Stopped using (OFF) + middle letters (heart of) de(AL)er = OFFAL (tripe as def by example = for instance), 14 Piece of cake (EASY) followed by (before) steak (as definition by example, maybe) of MEAT = EASY MEAT (someone gullible), 16 Anagram (re-recording) of USED GAIN = AGNUS DEI (choral piece by Samuel Barber). FYI gain is a technique in amplifying and recording music as this explains. https://www.dpamicrophones.com/mic-university/how-to-mic-a-choir, 18 Nothing (O) + unaltered (AS IS) = OASIS (spring), 20 Double definition of HAND, 21 Desire (ITCH) inserted into (to stop) (W)omen + talent (CRAFT) = WITCHCRAFT (misogynistic charge), 23 Anagram (rogue) of TRADERS = STARRED (was outstanding), 24 Middle letter (focus) of bo(R)is  + alternate letters (regularly) of jOhNsOn  contained in (absorbed by) wordplay (PUN) = PRONOUN (it’s what it is), 25 Singer (CHER) surrounding (accepts) fine (OK) = CHOKER (one of the stranglers), 26 (A)ir (D)efence + officer (DS) + raise (UP) = ADDS UP (is consistent). 

Down: 1 1st letter (start to) of (A)ttack (W)ith + A + outer letters (drawn) of (K)nif(E) = AWAKE (aroused), 2 Cancelled (OFF) follows (after)  springiness (GIVE) = (detected in) GIVE OFF (produce), 3 Character (SIGN) + joined (ALLiED) minus (missing) (I)ndividual = SIGNALLED (waved), 5 Hirsute (fURRY) with (H)usband in front (at the outset, deleting (don’t use) (f)orce = HURRY (haste), 6 Class (FORM) + Durham as example (perhaps) of UNI = UNIFORM (regular), 7 Vacation (HOLIday) surrounds (‘s about) (O)very, minus (cut short) 24hrs (day) + reversal (returning) of problem (SNAG) = HOOLIGANS (people behaving terribly), 10 Anagram (mixing) of SOME TONIC = COMES INTO (is left), 13 Dog (cORGI) minus 1st letter (heading off) + A + STICk minus last letter (ultimately getting lost) = ORGIASTIC (wild), 15 This place (HERE) inside (during) (S)pecial + Saturday evening broadcast initially (MOTD) = SMOTHERED (held in), 17 Downward (UNDER) + turn (GO) = UNDERGO (experience), 19 Call (RING) + (S)on following (after) 1st letters (first signs of) (S)erious (P)ersonal = ISSUES (springs), 21 Homophone (to listen to) of sport (wear) = WHERE (the place), 22 (F)ollowing + rather strange (RUM) + (P)riest = FRUMP (woman with little fashion sense.