Quantity of contents (3)
I believe the answer is:
'quantity' is the definition.
(ten is a kind of quantity)
'of contents' is the wordplay.
'of' indicates the answer is hidden within the clue.
'TEN' is hidden in the letters of 'contents'.
(Other definitions for ten that I've seen before include "Five times two" , "Roman numeral X" , "X-WORD" , "No of sides of a decagon" , "Half a score" .)