In time, break mayor's heart (6)
I believe the answer is:
trendy
'in' is the definition.
('in' can mean trendy or fashionable)
'time break mayor's heart' is the wordplay.
'time' becomes 't' (abbreviation).
'break' becomes 'rend' (I've seen this in other clues).
'heart' says to take the centre.
The centre of 'mayor' is 'y'.
't'+'rend'+'y'='TRENDY'
(Other definitions for trendy that I've seen before include "Smart" , "Modish" , "Less sensible; a dye plant" , "With-it" , "Stylish" .)