Infantryman pays for veteran heading for reunion (4,7)
I believe the answer is:
foot soldier
'infantryman' is the definition.
('foot soldier' can be a synonym of 'infantryman')
'pays for veteran heading for reunion' is the wordplay.
'pays' becomes 'foots' (footing is a kind of paying).
'for' says to put letters next to each other.
'veteran' becomes 'oldie' (I've seen this before).
'heading for' says to take the initial letters.
The first letter of 'reunion' is 'r'.
'foots'+'oldie'+'r'='FOOT-SOLDIER'
(Other definitions for foot soldier that I've seen before include "Valuable but lowly worker" , "Infantryman" .)