Heard animal whimper (4)
I believe the answer is:
mewl
'whimper' is the definition.
(synonyms)
'heard animal' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'animal' becomes 'mule' (offspring of a donkey and a horse).
'mule' sounds like 'MEWL'.
(Other definitions for mewl that I've seen before include "weakly cry" , "Cry feebly" , "Make a whimpering noise" , "Animal noise" , "Whimper (like a baby?)" .)