Vessel for gullible person (3)
I believe the answer is:
mug
This clue is a double definition.
'vessel' is the first definition.
(mug is a kind of vessel)
'gullible person' is the second definition.
(I've seen this before)
'for' acts as a link.
(Other definitions for mug that I've seen before include "Fool - drinking vessel" , "Slangy victim, by gum" , "Charlie" , "attack" , "Large drinking vessel or slang victim" .)