Time and again is in the enclosure (5)
I believe the answer is:
three
'enclosure' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'time and again is in the' is the wordplay.
'time and again' becomes 're' (I can't explain this - if you can you should believe this answer much more).
'is in' means one lot of letters goes inside another.
're' placed into 'the' is 'THREE'.
Can you help me to learn more?
(Other definitions for three that I've seen before include "trilogy" , "One, one, one" , "So many 12s today" , "Snooker score for potting green ball" , "Cardinal number" .)