Make sure friend is in a winning position (9)
I believe the answer is:
checkmate
'a winning position' is the definition.
'checkmate' can be an answer for 'position' (I have seen 'position of defeat' mean 'checkmate' so perhaps 'position' could also mean 'checkmate'). I'm unsure of the 'a winning' bit.
'make sure friend' is the wordplay.
'make sure' becomes 'check' (synonyms).
'friend' becomes 'mate' (mate is a kind of friend).
'check'+'mate'='CHECKMATE'
'is in' acts as a link.
Can you help me to learn more?
(Other definitions for checkmate that I've seen before include "The final killer move in chess" , "final defeat" , "The final winning move in chess" , "ending on board" , "Chess game ending" .)