In short cheese attracts insect (7)
I believe the answer is:
briefly
'in short' is the definition.
('briefly' can be a synonym of 'in short')
'cheese attracts insect' is the wordplay.
'cheese' becomes 'brie' (Brie is an example).
'attracts' means one lot of letters go next to another.
'insect' becomes 'fly' (fly is a kind of insect).
'brie'+'fly'='BRIEFLY'
(Other definitions for briefly that I've seen before include "Cursorily" , "Fleetingly" , "For a short time" , "gets to the point" , "In a nutshell" .)