American to fight roughly (5)
I believe the answer is:
about
'roughly' is the definition.
(about can mean roughly or approximately)
'american to fight' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'to fight' becomes 'bout' (as in a bout in boxing).
'a'+'bout'='ABOUT'
(Other definitions for about that I've seen before include "Circa" , "On the topic of" , "Almost; around" , "Concerning or around" , "Apropos, concerning" .)