Parasite invades ship and holds fast (6)
I believe the answer is:
sticks
'holds fast' is the definition.
('stick' can be a synonym of 'hold fast')
'parasite invades ship' is the wordplay.
'parasite' becomes 'tick' (type of parasite).
'invades' means one lot of letters goes inside another.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'tick' put within 'ss' is 'STICKS'.
'and' acts as a link.
(Other definitions for sticks that I've seen before include "Stands" , "Glues" , "Old fogeys" , "Twigs or thin branches of a tree" , "Refuses to move" .)