Worry mustang's tempted to bite (5)
I believe the answer is:
angst
'worry' is the definition.
(I've seen this before)
'mustang's tempted to bite' is the wordplay.
'to bite' indicates a hidden word (the answer has been bitten by the clue).
'ANGST' is hidden in the letters of 'mustangs tempted'.
(Other definitions for angst that I've seen before include "Feeling of guilt or remorse" , "Horrible feeling" , "Gnats (anag.)" , "uneasiness" , "Feeling of anxiety or guilt" .)