Alter vital access to capitals (5,3)
I believe the answer is:
shift key
'access to capitals' is the definition.
(on a computer keyboard)
'alter vital' is the wordplay.
'alter' becomes 'shift' (shifting is a kind of altering).
'vital' becomes 'key' (I've seen this in other clues).
'shift'+'key'='SHIFT-KEY'
(Other definitions for shift key that I've seen before include "Typewriter feature, used to achieve capital letters" , "Button on keyboard for capitals" , "One pressed to find capital" , "means of capital production" , "it produces capital" .)