Be ruthless — rotten to past love! (4,2,7)
I believe the answer is:
stop at nothing
'be ruthless' is the definition.
(I've seen this before)
'rotten to past love' is the wordplay.
'rotten' is an anagram indicator.
'love' becomes 'nothing' (I've seen this before**).
'to'+'past'='topast'
'topast' is an anagram of 'stopat'.
'stopat'+'nothing'='STOP AT NOTHING'
(Other definitions for stop at nothing that I've seen before include "3 [COUNTDOWN CONUMDRUM] will" , "Act ruthlessly" , "as announcers of rocket launches do?" , "Be totally ruthless" , "Ignore all scruples" .)