Increase, enhance (7)
I believe the answer is:
augment
I believe this clue is a double definition.
'increase' is the first definition.
(I know that augment is a more specific form of increase)
'enhance' is the second definition.
(I've seen this in another clue)
(Other definitions for augment that I've seen before include "Enlarge or increase" , "Add to, increase" , "Make greater in size, number, etc." , "Make larger" , "Grow - intensify" .)