Frequently become more mellow, having lost head (5)
I believe the answer is:
often
'frequently' is the definition.
('often' can be a synonym of 'frequently')
'become more mellow having lost head' is the wordplay.
'become more mellow' becomes 'soften'.
'having lost head' means to remove the first letter.
'soften' with its first letter taken away is 'OFTEN'.
(Other definitions for often that I've seen before include "Repeatedly, many times" , "on numerous occasions" , "A lot" , "occurring with frequency" , "Quite frequently" .)