Content to hold quantity (3)
I believe the answer is:
ten
'quantity' is the definition.
(ten is a kind of quantity)
'content to hold' is the wordplay.
'to hold' indicates the answer is hidden within the clue (I've seen 'holding' mean this).
'TEN' is hidden within 'content'.
(Other definitions for ten that I've seen before include "Half a score" , "Figure" , "time" , "Roman numeral X" , "X-WORD" .)