Understood vessel to be quiet (8)
I believe the answer is:
taciturn
'quiet' is the definition.
(thesaurus)
'understood vessel' is the wordplay.
'understood' becomes 'tacit' (I've seen this before).
'vessel' becomes 'urn' (urn is a kind of vessel).
'tacit'+'urn'='TACITURN'
'to be' is the link.
(Other definitions for taciturn that I've seen before include "Uncommunicative, of few words" , "Disinclined to speak" , "Without saying much" , "Disinclined towards conversation" , "Tight-lipped" .)