Read it quickly and began making it lighter, knowing this could be popular with dieters (7,4)
I believe the answer is:
skimmed milk
'quickly and began making it lighter knowing this could be popular with dieters' is the definition.
The definition suggests a singular noun which matches the answer.
'read it' is the wordplay.
I cannot quite see how this works, but
'read' could be 'skim' (skimming is a kind of reading) and 'skim' is located in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for skimmed milk that I've seen before include "Dairy product" , "Low-fat fluid" , "refrigerated product" .)