Plain success for old PM (7)
I believe the answer is:
baldwin
'old pm' is the definition.
'baldwin' can be an answer for 'pm' (I have seen 'Former PM' mean 'baldwin' so perhaps 'pm' could also mean 'baldwin'). I'm unsure of the 'old' bit.
'plain success' is the wordplay.
'plain' becomes 'bald' (both can mean unadorned).
'success' becomes 'win' (I've seen this in another clue).
'bald'+'win'='BALDWIN'
'for' is the link.
Can you help me to learn more?
(Other definitions for baldwin that I've seen before include "Medieval monarch - black American writer - British premier" , "Former PM" , "British PM three times between WWI and WWII" , "Tory leader" , "Prime minister" .)