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