Allow access to a circular enclosure (4)
I believe the answer is:
open
'allow access' is the definition.
I can't tell whether this defines the answer.
'circular enclosure' is the wordplay.
'circular' becomes 'o' (I've seen this in other clues).
'enclosure' becomes 'pen' (pen is a kind of enclosure).
'o'+'pen'='OPEN'
'to a' acts as a link.
Can you help me to learn more?
(Other definitions for open that I've seen before include "Sports contest" , "gaping" , "unsealed" , "Public" , "Allowing access" .)