Save barrel that’s not closed (3)
I believe the answer is:
but
'save' is the definition.
('but' can mean 'save' or 'except')
'barrel that's not closed' is the wordplay.
'barrel' becomes 'butt' (butt is a kind of barrel).
'that's not closed' means to remove the last letter.
'butt' with its final letter taken off is 'BUT'.
(Other definitions for but that I've seen before include "Conjunction meaningn on the other hand" , "Only; apart from" , "However" , "Except; nevertheless" , "Only in attribution" .)