Boxer dog — one with catalogue (8)
I believe the answer is:
pugilist
'boxer' is the definition.
(I know that boxer can be written as pugilist)
'dog one with catalogue' is the wordplay.
'dog' becomes 'pug' (pug is a kind of dog).
'one' becomes 'I' (Roman numeral).
'with' says to put letters next to each other.
'catalogue' becomes 'list' (a catalogue is a list of items).
'pug'+'i'+'list'='PUGILIST'
(Other definitions for pugilist that I've seen before include "Fist fighter" , "Prize fighter" , "A professional boxer" .)