Rugby player is put in terrible mire? That's unacceptable (8)
I believe the answer is:
improper
'unacceptable' is the definition.
(thesaurus)
'rugby player is put in terrible mire?' is the wordplay.
'rugby player' becomes 'prop' (type of rugby player).
'is put in' means one lot of letters goes inside another (back).
'terrible' indicates anagramming the letters.
'mire' is an anagram of 'imer'.
'prop' put within 'imer' is 'IMPROPER'.
'that's' is the link.
(Other definitions for improper that I've seen before include "Unseemly, indelicate" , "Indecent, unseemly" , "Incorrect or wrong, say behaviour" , "Unfitting, out of place" , "unfit" .)