Most aggrieved about animal (4)
I believe the answer is:
stag
'animal' is the definition.
(stag is a kind of animal)
'most aggrieved about' is the wordplay.
'about' says the answer is hidden in the clue (the clue goes around or about the answer).
'STAG' can be found hidden inside 'most aggrieved'.
(Other definitions for stag that I've seen before include "animal of the deer family" , "Male animal" , "without female escort" , "Male beast" , "Without women" .)