Unexpectedly tamed one parasitic worm (8)
I believe the answer is:
nematode
'parasitic worm' is the definition.
(nematode is a type of worm)
'unexpectedly tamed one' is the wordplay.
'unexpectedly' indicates anagramming the letters.
'tamed'+'one'='tamedone'
'tamedone' is an anagram of 'NEMATODE'.
(Other definitions for nematode that I've seen before include "sort of animal" , "Long thin worm such as threadworm" , "Type of worm" .)