Round enclosure, fully accessible (4)
I believe the answer is:
open
'fully accessible' is the definition.
'open' can be an answer for 'accessible' (something open can be accessed). I'm not sure about the 'fully' bit.
'round enclosure' is the wordplay.
'round' becomes 'o' (o is a circular letter).
'enclosure' becomes 'pen' (pen is a kind of enclosure).
'o'+'pen'='OPEN'
Can you help me to learn more?
(Other definitions for open that I've seen before include "Undo; exposed to attack" , "This championship includes both professional and amateur golfers" , "Golf tournament" , "William --, English poet" , "Frank - accessible" .)