Most of confirmation about volume is stipulation (7)
I believe the answer is:
proviso
'is stipulation' is the definition.
(I've seen this in another clue)
'most of confirmation about volume' is the wordplay.
'most of' means to remove the last letter.
'confirmation' becomes 'proof' (I've seen this in another clue).
'about' indicates putting letters inside.
'volume' becomes 'vis' (I can't explain this - if you can you should believe this answer much more).
'proof' with its last letter removed is 'proo'.
'proo' enclosing 'vis' is 'PROVISO'.
(Other definitions for proviso that I've seen before include "Condition for VIP or so" , "Condition added to an agreement" , "Rider" , "Condition attached to an agreement" , "Stipulation (in contract)" .)