Aggravate trendy sweetheart (7)
I believe the answer is:
inflame
'aggravate' is the definition.
(I know that inflame is a more specific form of the action aggravate)
'trendy sweetheart' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'sweetheart' becomes 'flame' (I've seen this in another clue).
'in'+'flame'='INFLAME'
(Other definitions for inflame that I've seen before include "Bother" , "Make (a situation) worse" , "Exacerbate" , "Intensify, aggravate" , "Excite passionate feelings" .)