It's good, say, for carrying water away (6)
I believe the answer is:
gutter
'carrying water away' is the definition.
I can't judge whether this definition defines the answer.
'it's good say' is the wordplay.
'it's good' becomes 'g' (abbreviation).
'say' becomes 'utter' (to utter something is to say or speak it).
'g'+'utter'='GUTTER'
'for' is the link.
Can you help me to learn more?
(Other definitions for gutter that I've seen before include "Channel for carrying off water" , "Element 46; a London theatre" , "Drainage channel" , "Channel running along the eaves of a building" , "Street channel for carrying off rainwater" .)