Grumble, as sheep in field might do (5)
I believe the answer is:
bleat
This clue is a double definition.
'grumble' is the first definition.
(both can mean to complain)
'as sheep in field might do' is the second definition.
(Other definitions for bleat that I've seen before include "Cry like a goat" , "Sheep's weak cry" , "Sound made by goat" , "Whine" , "Beef" .)