Modify top of dress during a fitting (6)
I believe the answer is:
adjust
'modify' is the definition.
(I've seen this before)
'top of dress during a fitting' is the wordplay.
'top of' indicates taking the first letters.
'during' means one lot of letters goes inside another.
'fitting' becomes 'just' (similar in meaning).
The initial letter of 'dress' is 'd'.
'a'+'just'='ajust'
'd' put into 'ajust' is 'ADJUST'.
(Other definitions for adjust that I've seen before include "Adapt, fix" , "Fine-tune" , "Tune" , "suit" , "Correct" .)