It may come back to one's ear (4)
I believe the answer is:
toot
'one's ear' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'it may come back to' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'may' becomes 'o' (I can't justify this - if you can you should believe this answer much more).
'come back' shows that the letters should be reversed in order.
'to' backwards is 'ot'.
't'+'o'+'ot'='TOOT'
Can you help me to learn more?
(Other definitions for toot that I've seen before include "Briefly blow" , "Short sound of horn" , "Brief blast on a horn" , "Short sounding of horn" , "Use horn" .)