Playful banter is hurtful in time (8)
I believe the answer is:
badinage
'playful banter' is the definition.
(badinage means cheerful banter)
'hurtful in time' is the wordplay.
'hurtful' becomes 'bad' (bad can mean hurtful or harmful).
'time' becomes 'age' (an age is a period of time).
'bad'+'in'+'age'='BADINAGE'
'is' is the link.
(Other definitions for badinage that I've seen before include "Backchat, banter" , "Raillery, give-and-take" , "Beg Diana with good-humoured teasing" , "Playful banter" , "Humorous, witty conversation" .)