Parasite chief shortly about to leave (6)
I believe the answer is:
chigoe
'parasite' is the definition.
(thesaurus)
'chief shortly about to leave' is the wordplay.
'shortly' means to remove the last letter.
'about' is an insertion indicator.
'to leave' becomes 'go' (to go is to depart or leave).
'chief' with its final letter removed is 'chie'.
'chie' enclosing 'go' is 'CHIGOE'.
(Other definitions for chigoe that I've seen before include "Indian flea which buries itself under toenails" , "Larva of skin-mite" .)