Grumbled (6)
I believe the answer is:
moaned
'grumbled' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for moaned that I've seen before include "Complained" , "A demon (anag.)" , "Made low sound of pain - or grumbled" , "Made low sound of pain or distress" , "Made low mournful sound as of pain" .)