Ready to complain, losing head (4)
I believe the answer is:
ripe
'ready' is the definition.
(similar in meaning)
'complain losing head' is the wordplay.
'complain' becomes 'gripe' (griping is a kind of complaining).
'losing head' means to remove the first letter.
'gripe' with its first letter taken away is 'RIPE'.
'to' acts as a link.
(Other definitions for ripe that I've seen before include "Pier (anag.)" , "Right" , "Ready for picking" , "Mature and ready to be eaten" , "Fully grown, ready for eating" .)