Heavyweight caught old American's boxing defeat (8)
I believe the answer is:
colossus
'heavyweight' is the definition.
(I know that heavyweight can be written as colossus)
'caught old american's boxing defeat' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'american' becomes 'US' (United States).
'boxing' indicates putting letters inside ('box' can mean contain, as in to put in a box).
'defeat' becomes 'loss' (I've seen this before).
'o'+'us'='ous'
'ous' enclosing 'loss' is 'olossus'.
'c'+'olossus'='COLOSSUS'
(Other definitions for colossus that I've seen before include "Large statue (of Rhodes?)" , "Gigantic statue that stood at the harbour entrance at Rhodes" , "Enormous figure" , "Something larger than life" , "Gigantic figure" .)