It features in botany and arithmetic (4)
I believe the answer is:
root
'it features' is the definition.
The definition suggests a singular noun which matches the answer.
'botany and arithmetic' is the wordplay.
'botany' becomes 'oot' (I can't explain this - if you can you should give a lot more credence to this answer).
'and' is a charade indicator (letters next to each other).
'arithmetic' becomes 'r' (one of the traditional three R's of education).
'oot' after 'r' is 'ROOT'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for root that I've seen before include "Reptile" , "Turn up with the snout." , "Derivation" , "Underground growth" , "Source; firmly establish" .)