Grumpy person (6)
I believe the answer is:
grouch
'grumpy person' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for grouch that I've seen before include "one known for grumbling" , "Grumble - complain" , "Person continually grumbling" , "Grumbler - gripe" , "Complain(ing person)" .)