Help to limit parrot-fish parasite (7)

I believe the answer is:
ascarid
'parasite' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Another definition for ascarid that I've seen is " parasite".)
I've seen this clue in The Sunday Times and The Times.
Want a hint initially instead of a full solution?
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