Fool nicking small wrench (5)
I believe the answer is:
twist
'wrench' is the definition.
('twist' can be a synonym of 'wrench')
'fool nicking small' is the wordplay.
'fool' becomes 'twit' (I've seen this before).
'nicking' means one lot of letters goes inside another (I've seen this in other clues).
'small' becomes 's' (abbreviation - e.g. clothes size).
'twit' placed around 's' is 'TWIST'.
(Other definitions for twist that I've seen before include "tobacco to chew?" , "Turn; cheat" , "Spiral" , "quirk?" , "Twine, coil" .)