Old friend causing impasse (9)
I believe the answer is:
stalemate
'impasse' is the definition.
('stalemate' can be a synonym of 'impasse')
'old friend' is the wordplay.
'old' becomes 'stale' ('stale' can be a synonym of 'old').
'friend' becomes 'mate' (mate is a kind of friend**).
'stale'+'mate'='STALEMATE'
'causing' is the link.
(Other definitions for stalemate that I've seen before include "Unresolvable position" , "Blocked position" , "Possible outcome of chess game" , "Deadlock, impasse" , "Inconclusive result" .)