Understood vessel could be reserved (8)
I believe the answer is:
taciturn
'be reserved' is the definition.
(I've seen this in another clue)
'understood vessel' is the wordplay.
'understood' becomes 'tacit' (I've seen this before).
'vessel' becomes 'urn' (urn is a type of container).
'tacit'+'urn'='TACITURN'
'could' acts as a link.
(Other definitions for taciturn that I've seen before include "Without saying much" , "Uncommunicative, of few words" , "Tending to silence" , "Habitually silent" , "Disinclined towards conversation" .)