Fullness of boxer that's hugged by tiny child (8)
I believe the answer is:
totality
'fullness' is the definition.
(thesaurus)
'boxer that's hugged by tiny child' is the wordplay.
'boxer' becomes 'ali' (Muhammad Ali).
'that's hugged by' is an insertion indicator (I've seen this in other clues).
'tiny child' becomes 'totty'.
'ali' going into 'totty' is 'TOTALITY'.
'of' acts as a link.
(Other definitions for totality that I've seen before include "Whole thing" , "Whole amount" , "Sum" , "Completeness" , "The complete number or amount" .)