Introduce round measures with variable size (9)
I believe the answer is:
immensity
'size' is the definition.
(I know that immensity is a type of bigness)
'introduce round measures with variable' is the wordplay.
I cannot really see how this works, but
'measures' could be 'ns' (N is an example) and 'ns' is present in the answer.
'variable' could be 'y' (algebra) and 'y' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for immensity that I've seen before include "bulk!" , "Hugeness" .)