Veteran hears row breaking out (8)
I believe the answer is:
warhorse
'veteran' is the definition.
('warhorse' can be a synonym of 'veteran')
'hears row breaking out' is the wordplay.
'breaking out' indicates an anagram.
'hears'+'row'='hearsrow'
'hearsrow' is an anagram of 'WARHORSE'.
(Other definitions for warhorse that I've seen before include "Old soldier" , "Veteran soldier" , "something too often played" , "Copenhagen, say" , "Familiar piece of music" .)