Twist key conclusion (4)
I believe the answer is:
bend
'twist' is the definition.
(synonyms)
'key conclusion' is the wordplay.
'key' becomes 'b' (eg B major, musical key).
'conclusion' becomes 'end' (something's conclusion is its end).
'b'+'end'='BEND'
(Other definitions for bend that I've seen before include "Turn - buckle" , "Crook" , "Crouch" , "submit" , "Curve; knot" .)