Adjust, fine-tune (5)
I believe the answer is:
tweak
I believe this clue is a double definition.
'adjust' is the first definition.
(I've seen this before)
'fine-tune' is the second definition.
(I've seen this in another clue)
(Other definitions for tweak that I've seen before include "Pinch and twist" , "Minor change" , "[TUG]" , "spin, thus?" , "Modify slightly" .)