Shock seeing potty upset (4)
I believe the answer is:
stun
'shock' is the definition.
('stun' can be a synonym of 'shock')
'potty upset' is the wordplay.
'potty' becomes 'nuts' (I've seen this before).
'upset' shows that the letters should be reversed in order.
'nuts' reversed gives 'STUN'.
'seeing' is the link.
(Other definitions for stun that I've seen before include "Stupefy - daze" , "Shock or astonish" , "Surprise greatly or daze" , "Daze, concuss" , "Concuss; amaze" .)