It makes a topping sweet (5,5)
I believe the answer is:
icing sugar
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'a topping sweet' is the wordplay.
'a topping' becomes 'icing' (icing is a kind of topping).
'sweet' becomes 'sugar' (I can't justify this - if you can you should give a lot more credence to this answer).
'icing'+'sugar'='ICING SUGAR'
'makes' acts as a link.
Can you help me to learn more?
(Other definitions for icing sugar that I've seen before include "Sweet stuff used for cake topping" , "Sweet powder used on cakes etc" , "Cake covering" .)