Dull friend reaches deadlock (9)
I believe the answer is:
stalemate
'deadlock' is the definition.
(I know that deadlock can be written as stalemate)
'dull friend' is the wordplay.
'dull' becomes 'stale'.
'friend' becomes 'mate' (mate is a kind of friend).
'stale'+'mate'='STALEMATE'
'reaches' is the link.
(Other definitions for stalemate that I've seen before include "Indecisive chess ending" , "situation without a winner" , "Possible chess game ending" , "An enforced draw in chess" , "A no-win situation in chess" .)