Save about litre in cask (6)
I believe the answer is:
barrel
'cask' is the definition.
('barrel' can be a synonym of 'cask')
'save about litre' is the wordplay.
'save' becomes 'bar' (I've seen this before).
'about' becomes 're' (regarding).
'litre' becomes 'l' ('l' can be a synonym of 'litre').
'bar'+'re'+'l'='BARREL'
'in' acts as a link.
(Other definitions for barrel that I've seen before include "Cask, drum" , "Large round container as for beer" , "VAT?" , "Oil/beer container" , "Container for beer etc" .)