Brief affair entertaining you at first (6)
I believe the answer is:
flying
'brief' is the definition.
(eg in a 'flying visit', a short visit)
'affair entertaining you at first' is the wordplay.
'affair' becomes 'fling' (I have seen 'Brief affair' mean 'fling' so perhaps 'affair' could also mean 'fling').
'entertaining' means one lot of letters goes inside another.
'at first' suggests taking the first letters.
The first letter of 'you' is 'y'.
'fling' placed around 'y' is 'FLYING'.
(Other definitions for flying that I've seen before include "Famous train once from King's Cross" , "''Those magnificent men in their ... machines'' (film theme song)" , "Taking to the air" , "- Boat; - bomb; - fox" , "-- Buttress; -- doctor" .)