Companion following Oswald is a parasite (5)
I believe the answer is:
leech
'parasite' is the definition.
(I know that parasite can be written as leech)
'companion following oswald' is the wordplay.
'companion' becomes 'ch' (Companion of Honour).
'following' is a charade indicator (letters next to each other).
'oswald' becomes 'lee' (I can't explain this - if you can you should believe this answer much more).
'ch' after 'lee' is 'LEECH'.
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for leech that I've seen before include "Worm - sponger" , "One who sponges on others" , "Blood-sucking creature" , "one good at sponging" , "One used to bleed" .)