Kid a boy it's added up (6)
I believe the answer is:
totted
'added up' is the definition.
('tot' can be a synonym of 'add up')
'kid a boy' is the wordplay.
'kid' becomes 'tot' (tot is a kind of kid).
'a boy' becomes 'ted' (short for Edward or Theodore).
'tot'+'ted'='TOTTED'
'it's' is the link.
(Other definitions for totted that I've seen before include "Added up, aggregated" , "Determine the sum of" .)