Means of calculating account in a computer system (6)
I believe the answer is:
abacus
'means of calculating' is the definition.
(an abacus is used to calculate things)
'account in a computer system' is the wordplay.
'account' becomes 'ac' (abbreviation for account).
'in' indicates putting letters inside.
'computer system' becomes 'bus' (computing term).
'a'+'bus'='abus'
'ac' put within 'abus' is 'ABACUS'.
(Other definitions for abacus that I've seen before include "means of multiplying" , "You could count on this once" , "Very early calculator" , "summer once?" , "Totter" .)