Make better or more attractive (7)
I believe the answer is:
enhance
'make better or more attractive' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for enhance that I've seen before include "'Improve, augment (7)'" , "Improve, add to attractiveness of" , "Raise in value or quality" , "Improve the value of (something)" , "Increase in power" .)