Dirty mask until person's changed (15)
I believe the answer is:
unsportsmanlike
'dirty' is the definition.
(synonyms)
'mask until person's changed' is the wordplay.
'changed' indicates anagramming the letters.
'mask'+'until'+'person'='maskuntilperson'
'maskuntilperson' is an anagram of 'UNSPORTSMANLIKE'.
(Other definitions for unsportsmanlike that I've seen before include "Sneaky" , "Dirty" , "Not cricket" , "as most female athletes are essentially?" , "Like going for a trip when playing football" .)