It stores gallons, say under a thousand (3)
I believe the answer is:
keg
'it stores gallons' is the definition.
(a keg might store gallons of drink)
'say under a thousand' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'under' says to put letters next to each other.
'a thousand' becomes 'k' (synonyms).
'eg' after 'k' is 'KEG'.
(Other definitions for keg that I've seen before include "Small cask or barrel" , "Barrel as for beer" , "What's found at King's Head" , "Small beer cask" , "Sort of beer" .)