Water carrier in gallons, say (6)
I believe the answer is:
gutter
'water carrier' is the definition.
(I've seen this in another clue)
'gallons say' is the wordplay.
'gallons' becomes 'G' (abbreviation).
'say' becomes 'utter' (to utter something is to say or speak it).
'g'+'utter'='GUTTER'
'in' is the link.
(Other definitions for gutter that I've seen before include "Drainage channel" , "Threaten to go out" , "Where snipe can be found" , "Roadside channel" , "Rain channel; (flame) start to die" .)