Parasite was first to be amused (7)
I believe the answer is:
tickled
'be amused' is the definition.
(I've seen this in another clue)
'parasite was first' is the wordplay.
'parasite' becomes 'tick' (type of parasite).
'was first' becomes 'led' (to lead is to go first).
'tick'+'led'='TICKLED'
'to' is the link.
(Other definitions for tickled that I've seen before include "Pleased" , "to be attracted to poacher" , "Delighted" , "Amused" , "Made to giggle" .)