Initially dirty water channel? (5)
I believe the answer is:
drain
'channel?' is the definition.
(drain is a kind of channel)
'initially dirty water' is the wordplay.
'initially' says to take the initial letters.
'water' becomes 'rain' (rain is a kind of water).
The first letter of 'dirty' is 'd'.
'd'+'rain'='DRAIN'
(Other definitions for drain that I've seen before include "See 11 dn" , "Wasted down here" , "Culvert" , "Gutter" , "To empty (of fluid)" .)