Seen to contain five in number (5)
I believe the answer is:
seven
'number' is the definition.
(seven is a kind of whole number)
'seen to contain five' is the wordplay.
'to contain' indicates putting letters inside.
'five' becomes 'V' (Roman numeral).
'seen' placed around 'v' is 'SEVEN'.
'in' acts as a link.
(Other definitions for seven that I've seen before include "Figure of dwarfs?" , "Number (of stars in the sky?)" , "number of swans aswimming" , "See 1" , "Number of sins, seas or dwarves" .)