Fine-tune; adjust (5)
I believe the answer is:
tweak
This clue is a double definition.
'fine-tune' is the first definition.
(I've seen this in another clue)
'adjust' is the second definition.
(I've seen this in another clue)
(Other definitions for tweak that I've seen before include "Pull and twist" , "Make small adjustments" , "Fine-tune, twist" , "Modify slightly" , "Pinch sharply" .)