Intensify, increase (8)
I believe the answer is:
heighten
I believe this clue is a double definition.
'intensify' is the first definition.
(I know that heighten is a more specific form of intensify)
'increase' is the second definition.
(I know that heighten is a more specific form of increase)
(Other definitions for heighten that I've seen before include "effect some improvement" , "Move up" , "Raise" , "Elevate - intensify" , "Become more intense" .)