Unfit rugby player stuck in dreadful mire (8)
I believe the answer is:
improper
'unfit' is the definition.
(thesaurus)
'rugby player stuck in dreadful mire' is the wordplay.
'rugby player' becomes 'prop' (I've seen this in other clues).
'stuck in' is an insertion indicator.
'dreadful' indicates anagramming the letters.
'mire' with letters rearranged gives 'imer'.
'prop' going into 'imer' is 'IMPROPER'.
(Other definitions for improper that I've seen before include "vulgar" , "Incorrect or wrong, say behaviour" , "Not in line with accepted standards of morality" , "Hardly meet" , "Unfitting, out of place" .)