This action, involving parasite's eggs, on reflection, is unavoidable (8)
I believe the answer is:
destined
'unavoidable' is the definition.
(thesaurus)
'this action involving parasite's eggs on reflection' is the wordplay.
'this action' becomes 'deed' (deed is a kind of action).
'involving' is an insertion indicator.
'parasite's eggs' becomes 'nits' (I can't explain this - if you can you should believe this answer much more).
'on reflection' is a reversal indicator.
'deed' going around 'nits' is 'denitsed'.
'denitsed' backwards is 'DESTINED'.
'is' is the link.
Can you help me to learn more?
(Other definitions for destined that I've seen before include "Fated, fore-ordained" , "Intended; doomed" , "Didn't see, being fated" , "Designated" , "Intended (for a particular end)" .)