Fight roughly after losing head (4)
I believe the answer is:
'fight' is the definition.
(as in a bout in boxing)
'roughly after losing head' is the wordplay.
'roughly' becomes 'about' (about can mean roughly or approximately).
'after losing head' means to remove the first letter.
'about' with its initial letter removed is 'BOUT'.
(Other definitions for bout that I've seen before include "Attack (of illness)" , "Fit" , "Contest between boxers" , "Period of activity or boxing contest" , "period in ring" .)