Parasite's brief panic about pound (4)
I believe the answer is:
flea
'parasite's' is the definition.
(I have seen 'Tiny parasite ' mean 'flea' so perhaps 'parasite' could also mean 'flea')
'brief panic about pound' is the wordplay.
'brief' means to remove the last letter (a shorter or briefer version of the word).
'panic' becomes 'fear' (I've seen this in another clue).
'about' is an insertion indicator.
'pound' becomes 'L' (abbreviation e.g. Lsd - pounds, shillings, pence).
'fear' with its final letter removed is 'fea'.
'fea' going around 'l' is 'FLEA'.
(Other definitions for flea that I've seen before include "source of infestation" , "Circus insect?" , "Irritating jumper?" , "Jumping, blood-sucking insect" , "one is proverbially fit" .)