Some in Fine Gael faction upset by one of the parasites (4)
I believe the answer is:
flea
'one of the parasites' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'some in fine gael faction upset' is the wordplay.
'some in' says the answer is hidden in the clue.
'upset' says the letters should be written in reverse.
'aelf' can be found hidden inside 'fine gael faction'.
'aelf' back-to-front is 'FLEA'.
'by' acts as a link.
Can you help me to learn more?
(Other definitions for flea that I've seen before include "Irritating jumper?" , "Small jumper in small circus" , "Bloodsucking parasite" , "Tiny parasite" , "Jumping bloodsucker" .)