Grumble, complain (6)
I believe the answer is:
grouch
I believe this clue is a double definition.
'grumble' is the first definition.
(I know that grumble can be written as grouch)
'complain' is the second definition.
(I know that grouch is a more specific form of the action complain)
(Other definitions for grouch that I've seen before include "Complain(ing person)" , "Grumble - complain" , "Bellyache" , "carp" , "'Crosspatch, grump (6)'" .)