Nag's sounding off — wife cross (6)
I believe the answer is:
whinny
'nag's sounding off' is the definition.
I can't tell whether this defines the answer.
'wife cross' is the wordplay.
'wife' becomes 'w' (abbreviation).
'cross' becomes 'hinny' (I can't explain this - if you can you should give a lot more credence to this answer).
'w'+'hinny'='WHINNY'
Can you help me to learn more?
(Other definitions for whinny that I've seen before include "Joyful neigh of a horse" , "Soft neigh or laugh" , "Gentle or joyful neigh" , "Beastly sound" , "High-pitched neigh" .)