Banter becomes hurtful in time (8)
I believe the answer is:
badinage
'banter' is the definition.
(I know that badinage is a type of 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'
'becomes' is the link.
(Other definitions for badinage that I've seen before include "Frivolous banter" , "Raillery, give-and-take" , "Beg Diana for light banter" , "A big Dane given to banter" , "Teasing talk, banter" .)