It often involves unknown quantities (7)
I believe the answer is:
algebra
'it often involves unknown quantities' is the definition.
The definition suggests a singular noun which matches the answer.
This is the entire clue.
Can you help me to learn more?
(Other definitions for algebra that I've seen before include "Branch of generalised arithmetic" , "Calculation by symbols" , "Symbolic mathematics" , "Maths with letters representing numbers" , "Branch of maths using letters and symbols" .)