Turn key to finish (4)
I believe the answer is:
bend
'turn' is the definition.
(I know that turn can be written as bend)
'key to finish' is the wordplay.
'key' becomes 'b' (eg B major, musical key).
'to finish' becomes 'end' ('end' can be a synonym of 'finish').
'b'+'end'='BEND'
(Other definitions for bend that I've seen before include "Corner" , "Lean forward" , "Curve; knot" , "Shape into a curve" , "Influence, bow" .)