In time, hesitation can be fun (6)
I believe the answer is:
humour
'fun' is the definition.
(I know that humour is a type of fun)
'in time hesitation' is the wordplay.
'in' means one lot of letters goes inside another.
'time' becomes 'hour' (unit of time).
'hesitation' becomes 'um' (um is said in hesitation).
'hour' placed around 'um' is 'HUMOUR'.
'can be' is the link.
(Other definitions for humour that I've seen before include "Temperament; comedy" , "Hilarity" , "A funny thing" , "Bodily fluid" , "See 12" .)