Turn bishop to conclusion (4)
I believe the answer is:
bend
'turn' is the definition.
(bending is a kind of turning)
'bishop to conclusion' is the wordplay.
'bishop' becomes 'b' (abbreviation used in chess).
'to conclusion' becomes 'end' (end is a kind of conclusion).
'b'+'end'='BEND'
(Other definitions for bend that I've seen before include "Turn, flex" , "Arc" , "Lean forward" , "elbow" , "Influence, bow" .)