Conduit for a fluid (4)
I believe the answer is:
duct
'conduit for a fluid' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for duct that I've seen before include "Tube for cables, air, etc" , "Air passage" , "Pipe conveying fluid or gas" , "The Eve of St - is a narrative poem by Keats" , "Tube for conveing cable, liquid etc" .)