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 am not sure about this - if you are sure you should believe this answer much more).
'out' is an insertion indicator.
'v' going 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" .)