Not saying much but understood on the vessel (8)
I believe the answer is:
taciturn
'not saying much' is the definition.
(I've seen this before)
'understood on the vessel' is the wordplay.
'understood' becomes 'tacit' (I've seen this before).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'the vessel' becomes 'urn' (urn is a type of container).
'tacit'+'urn'='TACITURN'
'but' is the link.
(Other definitions for taciturn that I've seen before include "Without saying much" , "Tight-lipped" , "Aloof" , "Reserved and uncommunicative" , "Disinclined towards conversation" .)