Lively bishop joins two knights in retreat (5)
I believe the answer is:
brisk
'lively' is the definition.
(synonyms)
'bishop joins two knights in retreat' is the wordplay.
'two knights' means two replacements for 'knight'.
'bishop' becomes 'B' (chess abbreviation).
'joins' is a charade indicator (letters next to each other).
'knight' becomes 'k'.
'knight' becomes 'sir' (a knight has the title Sir).
'in retreat' is a reversal indicator.
'k'+'sir'='ksir'
'ksir' reversed gives 'risk'.
'b'+'risk'='BRISK'
(Other definitions for brisk that I've seen before include "Lively, invigorating" , "Quick, active, energetic" , "Spry" , "Bustling" , "Quick and invigorating" .)