Circular letter with enclosure, unsealed (4)
I believe the answer is:
open
'unsealed' is the definition.
('open' can be similar in meaning to 'unsealed')
'circular letter with enclosure' is the wordplay.
'circular letter' becomes 'o'.
'with' is a charade indicator (letters next to each other).
'enclosure' becomes 'pen' (pen is a kind of enclosure).
'o'+'pen'='OPEN'
(Other definitions for open that I've seen before include "with no clear favourite" , "Frank - unsettled" , "simple" , "Frank; unobstructed" , "yawning" .)