Got in hot pants, resolved to be ruthless (4,2,7)
I believe the answer is:
stop at nothing
'be ruthless' is the definition.
(I've seen this in another clue)
'got in hot pants resolved' is the wordplay.
'resolved' indicates an anagram.
'got'+'in'+'hot'+'pants'='gotinhotpants'
'gotinhotpants' is an anagram of 'STOP AT NOTHING'.
'to' acts as a link.
(Other definitions for stop at nothing that I've seen before include "as announcers of rocket launches do?" , "Act ruthlessly" , "Be totally ruthless" , "Ignore all scruples" , "3 [COUNTDOWN CONUMDRUM] will" .)