Gross expression of pain - not a happy bunny (6)
I believe the answer is:
grouch
'not a happy bunny' is the definition.
(both can mean a grumpy person)
'gross expression of pain' is the wordplay.
'gross' becomes 'gr' (I've seen this in other clues).
'expression of pain' becomes 'ouch' (I've seen this before).
'gr'+'ouch'='GROUCH'
(Other definitions for grouch that I've seen before include "Habitually grumpy person" , "one known for grumbling" , "Complaint" , "carp" , "Bellyache" .)