Veteran sent up guns in unopened case (3,6)
I believe the answer is:
old stager
'veteran' is the definition.
(synonyms)
'sent up guns in unopened case' is the wordplay.
'sent up' says the letters should be written backwards.
'guns' becomes 'gats' (gat is a kind of gun).
'in' indicates putting letters inside.
'unopened' means to remove the first letter.
'case' becomes 'holder'.
'gats' backwards is 'stag'.
'holder' with its initial letter removed is 'older'.
'stag' inserted within 'older' is 'OLD STAGER'.
(Other definitions for old stager that I've seen before include "One who's experienced a lot" , "expert" , "senior citizen" , "veteran" .)