Change plug perfectly (6)
I believe the answer is:
adjust
'change' is the definition.
(to adjust is to change)
'plug perfectly' is the wordplay.
'plug' becomes 'ad' (as in plugging a product).
'perfectly' becomes 'just' (as in 'just so').
'ad'+'just'='ADJUST'
(Other definitions for adjust that I've seen before include "Modify, adapt" , "Change slightly to fit" , "Calibrate or regulate" , "Modify - tweak" , "Fine-tune" .)