Vessel at sea radioed request (4)
I believe the answer is:
cask
'vessel' is the definition.
(cask is a kind of vessel)
'sea radioed request' is the wordplay.
I cannot quite see how this works, but
'request' could be 'ask' (to ask is to request something) and 'ask' is found within the answer.
A single letter 'c' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'at' is the link.
Can you help me to learn more?
(Other definitions for cask that I've seen before include "VAT?" , "Wine vessel; sack (anag.)" , "Liquor container" , "Wine barrel" , "Beer container" .)