Heighten, augment (7)
I believe the answer is:
enhance
This clue is a double definition.
'heighten' is the first definition.
(I know that enhance is a kind of heighten)
'augment' is the second definition.
(I've seen this before)
(Other definitions for enhance that I've seen before include "Intensify, increase or improve" , "improve things" , "Increase in power" , "Make better or more attractive" , "Improve the value of (something)" .)