Vessel in underground channel heading off (4)
I believe the answer is:
ewer
'vessel' is the definition.
(ewer is a kind of vessel)
'underground channel heading off' is the wordplay.
'underground channel' becomes 'sewer' (I've seen this in another clue).
'heading off' means to remove the first letter.
'sewer' with its initial letter taken away is 'EWER'.
'in' acts as a link.
(Other definitions for ewer that I've seen before include "Vessel for pouring" , "from which liquid may be poured" , "A wide-mouthed water jug" , "water-carrier" , "A type of jug" .)