Calculate when child out of bed (3,2)
I believe the answer is:
tot up
'calculate' is the definition.
'child out of bed' is the wordplay.
'child' becomes 'tot' (term for a young child).
'out of bed' becomes 'up' (synonyms).
'tot'+'up'='TOT UP'
'when' acts as a link.
(Other definitions for tot up that I've seen before include "Summate" , "Tally" , "Count" , "Add together (driving offences)" , "Determine the sum of" .)