Number five checked out (5)
I believe the answer is:
seven
'number' is the definition.
(seven is a kind of number)
'five checked out' is the wordplay.
'five' becomes 'V' (Roman numeral).
'checked' becomes 'seen' (I can't explain this - if you can you should give a lot more credence to this answer).
'out' is an insertion indicator.
'v' placed into 'seen' is 'SEVEN'.
Can you help me to learn more?
(Other definitions for seven that I've seen before include "equivalent of uproar [7D]" , "Film" , "number of swans aswimming" , "- Against Thebes (Aeschylus)" , "Prime number" .)