Cry: cross by the sound of it? (4)
I believe the answer is:
mewl
'cry' is the definition.
(mewling is a kind of crying)
'cross by the sound of it?' is the wordplay.
'cross' becomes 'mule' (a mule is a cross of a donkey and a horse).
'by the sound of it?' shows a homophone (sound like).
'mule' is a homophone of 'MEWL'.
(Other definitions for mewl that I've seen before include "weakly cry" , "Animal noise" , "Make a whimpering noise" , "Cry feebly" , "Whimper (like a baby?)" .)