Understood vase to be reserved (8)
I believe the answer is:
taciturn
'be reserved' is the definition.
(I've seen this before)
'understood vase' is the wordplay.
'understood' becomes 'tacit' (I've seen this before).
'vase' becomes 'urn' (urn is a kind of vase).
'tacit'+'urn'='TACITURN'
'to' is the link.
(Other definitions for taciturn that I've seen before include "Disinclined to speak" , "Tight-lipped" , "Far from talkative" , "Habitually silent" , "Tending to silence" .)