There's a lot of interest taken in it (5)
I believe the answer is:
usury
'it' is the definition.
The definition suggests a singular noun which matches the answer.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for usury that I've seen before include "Taking a lot of interest" , "Lending money at excessive rates of interest" , "Shylock's calling" , "Too much interest" , "Lending at high interest rates" .)