Understood vase is rarely open (8)
I believe the answer is:
taciturn
'rarely open' is the definition.
'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'
'is' is the link.
(Other definitions for taciturn that I've seen before include "Disinclined to speak" , "Aloof" , "Uncommunicative, of few words" , "Tight-lipped" , "Talking little" .)