Unfit forward stuck in horrid mire (8)
I believe the answer is:
improper
'unfit' is the definition.
(thesaurus)
'forward stuck in horrid mire' is the wordplay.
'forward' becomes 'prop' (I've seen this before).
'stuck in' indicates putting letters inside.
'horrid' indicates an anagram.
'mire' anagrammed gives 'imer'.
'prop' placed into 'imer' is 'IMPROPER'.
(Other definitions for improper that I've seen before include "Inappropriate" , "Not conforming to norms of conduct, maybe indecent" , "vulgar" , "Not decent, unseemly" , "Not in accord with norms of behaviour" .)