Paul's companion, spineless parasite blowing top (4)
I believe the answer is:
luke
'paul's companion' is the definition.
(Biblical figures)
'spineless parasite blowing top' is the wordplay.
'spineless parasite' becomes 'fluke' (fluke worm).
'blowing top' means to remove the first letter.
'fluke' with its first letter taken off is 'LUKE'.
(Other definitions for luke that I've seen before include "One of the four Evangelists" , "Biblical evangelist" , "Physician gospeller" , "Patron saint of physicians" , "Third NT book" .)