Frequently become milder after losing head (5)
I believe the answer is:
'frequently' is the definition.
'become milder after losing head' is the wordplay.
'become milder' becomes 'soften' (to soften is to become milder or more mellow).
'after losing head' means to remove the first letter.
'soften' with its initial letter taken off is 'OFTEN'.
(Other definitions for often that I've seen before include "Frequently, not seldom" , "Frequently, habitually" , "Multiply" , "Perhaps daily" , "Frequent" .)