Old boxer, a large one (3)
I believe the answer is:
ali
'old boxer' is the definition.
(Muhammad Ali)
'a large one' is the wordplay.
'large' becomes 'l'.
'one' becomes 'i' (Roman numeral**).
'a'+'l'+'i'='ALI'
(Other definitions for ali that I've seen before include "Muslim name" , "Fourth caliph of Islam" , "Muhammad ..., famous boxer" , "Lepidopterous floater, apian stinger, d. 2016" , "Muhammad --, US boxer" .)