Water carrier underneath opening of storm drain (5)
I believe the answer is:
sewer
'drain' is the definition.
(I know that sewer is a type of drain)
'water carrier underneath opening of storm' is the wordplay.
'water carrier' becomes 'ewer' (I've seen this in another clue).
'underneath' means one lot of letters go next to another (in a down clue, some letters go under others).
'opening of' suggests taking the first letters.
The initial letter of 'storm' is 's'.
'ewer' after 's' is 'SEWER'.
(Other definitions for sewer that I've seen before include "Drainage channel" , "Waste-pipe" , "Pipe for waste matter from houses, factories etc" , "Underground waste pipe" , "Underground drain" .)