Sell important means to create capital (5,3)
I believe the answer is:
shift key
'means to create capital' is the definition.
(computer key used when typing capital letters)
'sell important' is the wordplay.
'sell' becomes 'shift' (shift can mean to get rid of or sell).
'important' becomes 'key' (similar in meaning).
'shift'+'key'='SHIFT-KEY'
(Other definitions for shift key that I've seen before include "Press it to type capitals" , "Typewriter feature, used to achieve capital letters" , "It's pressed while making THIS" , "it produces capital" , "Use it to move from lower-case to upper-case letters" .)