Join a fight, roughly (5)
I believe the answer is:
about
'roughly' is the definition.
('about' can be a synonym of 'roughly')
'join a fight' is the wordplay.
'join' means one lot of letters go next to another.
'fight' becomes 'bout' ('bout' can be a synonym of 'fight').
'a' put next to 'bout' is 'ABOUT'.
(Other definitions for about that I've seen before include "Approximately, around" , "Circa" , "Engaged in" , "Concerning; roughly" , "Concerning; approximately" .)