The sum to put back in incomplete account (5)
I believe the answer is:
total
'the sum' is the definition.
('total' can be a synonym of 'sum')
'to put back in incomplete account' is the wordplay.
'put back' is a reversal indicator.
'in' means one lot of letters goes inside another.
'incomplete' means to remove the last letter.
'account' becomes 'tale' (tale is a kind of account).
'to' in reverse letter order is 'ot'.
'tale' with its last letter taken off is 'tal'.
'ot' going within 'tal' is 'TOTAL'.
(Other definitions for total that I've seen before include "Complete, absolute" , "Whole, complete" , "to give the final figure" , "Not qualified" , "Completely destroy" .)