Nematode (9)
'nematode' is the definition.
(synonyms)
This is the entire clue.
(Another definition for roundworm that I've seen is " parasite perhaps".)
'nematode' is the definition.
(eelworm is a kind of nematode)
This is all the clue.
(Other definitions for eelworm that I've seen before include "Troublesome nematode" , "Parasitic threat to crops" , "parasite" .)