Roughly, it's a fit (5)
I believe the answer is:
about
'roughly' is the definition.
(about can mean roughly or approximately)
'a fit' is the wordplay.
'fit' becomes 'bout' (I've seen this before).
'a'+'bout'='ABOUT'
'it's' is the link.
(Other definitions for about that I've seen before include "On the topic of" , "'All - - Eve' (film)" , "Moving" , "Regarding" , "Apropos, concerning" .)