Quantity of works with no date (7)
I believe the answer is:
operand
'quantity' is the definition.
(I know that operand is a type of quantity)
'works with no date' is the wordplay.
'works' becomes 'opera' ('opera' is an alternative plural of 'opus').
'with' says to put letters next to each other.
'no date' becomes 'nd'.
'opera'+'nd'='OPERAND'
'of' is the link.
(Other definitions for operand that I've seen before include "Mathematical quantity altered by a process" , "Quantity to be worked on (maths.)" .)