A guard for the money? Go on! (6)
I believe the answer is:
sentry
'a guard' is the definition.
(sentry is a kind of guard)
'the money? go on' is the wordplay.
'the money?' becomes 'sen' (I've seen this before**).
'go' becomes 'try' (I've seen this before).
'on' means one lot of letters go next to another.
'sen'+'try'='SENTRY'
'for' is the link.
(Other definitions for sentry that I've seen before include "One keeps watch" , "Military guard" , "Lookout person" , "Sentinel, look-out" , "Soldier posted to keep watch" .)