Irritating little thing Knocking over a fairy (4)
I believe the answer is:
flea
'irritating little thing knocking' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'over a fairy' is the wordplay.
'over' says the letters should be written backwards.
'fairy' becomes 'elf' (elf is a kind of fairy).
'a'+'elf'='aelf'
'aelf' reversed gives 'FLEA'.
Can you help me to learn more?
(Other definitions for flea that I've seen before include "high-jumper" , "Irritating jumper?" , "beastie?" , "Bug" , "In one's ear, it's a stinging rebuke" .)