Quantity missing initial rise (5)
I believe the answer is:
mount
'rise' is the definition.
(both can mean to increase)
'quantity missing initial' is the wordplay.
'quantity' becomes 'amount' (amount is a kind of quantity).
'missing initial' means to remove the first letter.
'amount' with its initial letter taken off is 'MOUNT'.
(Other definitions for mount that I've seen before include "St. Helens, for example" , "Climb - eg Everest" , "Get on - horse" , "Get on top of; frame" , "Increase" .)