Unguarded ring, enclosure (4)
I believe the answer is:
open
'unguarded' is the definition.
(I've seen this in another clue)
'ring enclosure' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'enclosure' becomes 'pen' (pen is a kind of enclosure).
'o'+'pen'='OPEN'
(Other definitions for open that I've seen before include "Undo; exposed to attack" , "William --, English poet" , "Allowing entry and exit" , "Contest that includes all comers" , "sporting competition" .)