It's good, say, as a water carrier (6)
I believe the answer is:
gutter
'a water carrier' is the definition.
(I've seen this in another clue)
'it's good say' is the wordplay.
'it's good' becomes 'g' (abbreviation).
'say' becomes 'utter' (to utter something is to speak or say it).
'g'+'utter'='GUTTER'
'as' acts as a link.
(Other definitions for gutter that I've seen before include "Street channel for carrying off rainwater" , "Threaten to go out" , "Element 46; a London theatre" , "Where snipe can be found" , "Channel for carrying off water" .)