Close, as a vessel? (5)
I believe the answer is:
muggy
'close' is the definition.
(I've seen this before)
'a vessel?' is the wordplay.
I cannot quite see how this works, but
'vessel?' could be 'mug' (mug is a kind of vessel) and 'mug' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'as' acts as a link.
Can you help me to learn more?
(Other definitions for muggy that I've seen before include "Oppressively humid" , "Damp and stifling" , "close" , "Warm and humid" , "Unpleasantly humid" .)