It's required in hearing! (3)
I believe the answer is:
ear
'hearing' is the definition.
(ear is a kind of hearing)
'it's required' is the wordplay.
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'required' becomes 'ar' (I can't justify this - if you can you should give a lot more credence to this answer).
'e'+'ar'='EAR'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for ear that I've seen before include "Go astray" , "Musical appreciation" , "See 8" , "Ability to recognise sounds" , "good sense of music" .)