Computer accessory's second purpose (5)
I believe the answer is:
mouse
'computer accessory's' is the definition.
(I've seen this before)
'second purpose' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'purpose' becomes 'use' (synonyms).
'mo'+'use'='MOUSE'
(Other definitions for mouse that I've seen before include "Computer accessory" , "Proverbially quiet rodent" , "Squeaky creature" , "One sitting by computer" , "whose movements are captured on screen" .)