Number of contents (3)
I believe the answer is:
ten
'number' is the definition.
(ten is a kind of whole number)
'of contents' is the wordplay.
'of' indicates a hidden word.
'TEN' can be found hidden inside 'contents'.
(Other definitions for ten that I've seen before include "Figure" , "Roman numeral X" , "No of sides of a decagon" , "How many" , "Base of decimal system" .)