Unfit rugby player stuck in dreadful mire (8)
I believe the answer is:
Here is my best explanation:
'unfit' is the definition.
'rugby player stuck in dreadful mire' is the subsidiary indication.
'rugby player' becomes 'prop' (I've seen this in other clues).
'stuck in' indicates putting letters inside.
'dreadful' is an anagram indicator.
'mire' is an anagram of 'imer'.
'prop' put into 'imer' is 'improper'.
(Other definitions for improper that I've seen before include "Not in accord with norms of behaviour", "'Not suitable, right or appropriate (8)'", "Unsuitable", "vulgar", "Not fitting", "Unseemly, indelicate", "Indecent, unseemly", "Unseemly; (fraction) more than one", "Unfitting, out of place" and "So it may appear a fraction".)