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