End of game sees friend following regular pattern (9)
I believe the answer is:
checkmate
'end of game' is the definition.
(end of a game of chess)
'friend following regular pattern' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend).
'following' is a charade indicator (letters next to each other).
'regular pattern' becomes 'check' (a checked pattern).
'mate' put after 'check' is 'CHECKMATE'.
'sees' is the link.
(Other definitions for checkmate that I've seen before include "Where the king can't move" , "The final killer move in chess" , "position of defeat" , "Complete victory" , "Declaration of chess victory" .)