Young child settled unknown sum (8)
I believe the answer is:
totality
'sum' is the definition.
(I know that sum can be written as totality)
'young child settled unknown' is the wordplay.
'young child' becomes 'tot' (tot can mean a baby or toddler).
'settled' becomes 'alit' (I've seen this before).
'unknown' becomes 'y' (algebra).
'tot'+'alit'+'y'='TOTALITY'
(Other definitions for totality that I've seen before include "The whole of something" , "The complete number or amount" , "Sum" , "Whole thing" , "Tilt a toy (anag.)" .)