Number seen to include a five (5)
I believe the answer is:
seven
'number' is the definition.
(I know that seven is a type of digit)
'seen to include a five' is the wordplay.
'to include' means one lot of letters goes inside another.
'a five' becomes 'V' (Roman numeral).
'seen' placed around 'v' is 'SEVEN'.
(Other definitions for seven that I've seen before include "Theme of exotic dance" , "Number of sins, seas or dwarves" , "Cube root of 343" , "Wonders of the World - Deadly Sins" , "- Against Thebes (Aeschylus)" .)