Turn a nut with a key to make adjustment (4)
I believe the answer is:
tune
'adjustment' is the definition.
(I know that tune is a type of adjustment)
'turn a nut with a key' is the wordplay.
'turn' shows that the letters should be reversed in order.
'with' is a charade indicator (letters next to each other).
'a key' becomes 'e' (musical key).
'nut' written backwards gives 'tun'.
'tun'+'e'='TUNE'
'to make' acts as a link.
(Other definitions for tune that I've seen before include "Song" , "Optimise the condition of" , "temper" , "Improve quality" , "Air, melody" .)