Parasite - what a coincidence (5)
I believe the answer is:
fluke
'coincidence' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for fluke that I've seen before include "Parasitic flatworm" , "Lucky happening" , "Lucky chance" , "Lucky shot" , "Lucky stroke - part of anchor" .)