Against mission forgetting saint of great age (7)
I believe the answer is:
antique
'of great age' is the definition.
(I've seen this before)
'against mission forgetting saint' is the wordplay.
'against' becomes 'anti' (prefix meaning against).
'mission' becomes 'quest' (a quest is a mission or search).
'forgetting' is a deletion indicator.
'saint' becomes 'st' (abbreviation).
'quest' with 'st' removed is 'que'.
'anti'+'que'='ANTIQUE'
(Other definitions for antique that I've seen before include "Old collectable item" , "Old-fashioned, outdated" , "Long-standing" , "Old item of furniture" , "Lovely old piece" .)