PM unlocked success? (7)
I believe the answer is:
baldwin
'pm' is the definition.
(I have seen 'Former PM' mean 'baldwin' so perhaps 'pm' could also mean 'baldwin')
'unlocked success?' is the wordplay.
'unlocked' becomes 'bald' (without locks of hair).
'success?' becomes 'win' (I've seen this in another clue).
'bald'+'win'='BALDWIN'
(Other definitions for baldwin that I've seen before include "Tory leader" , "Medieval monarch - black American writer - British premier" , "British PM three times between WWI and WWII" , "Former PM" , "Prime minister" .)