Dull friend in a cul-de-sac (9)
I believe the answer is:
stalemate
'a cul-de-sac' is the definition.
(thesaurus)
'dull friend' is the wordplay.
'dull' becomes 'stale'.
'friend' becomes 'mate' (mate is a kind of friend).
'stale'+'mate'='STALEMATE'
'in' is the link.
(Other definitions for stalemate that I've seen before include "where moving is impossible" , "Indecisive chess ending" , "Inconclusive result" , "Blocked position" , "Deadlock, impasse" .)