Parasite went ahead, excited (7)
I believe the answer is:
tickled
'excited' is the definition.
(tickling is a kind of exciting)
'parasite went ahead' is the wordplay.
'parasite' becomes 'tick' (type of parasite).
'went ahead' becomes 'led' (led the way).
'tick'+'led'='TICKLED'
(Other definitions for tickled that I've seen before include "Delighted" , "Amused" , "Pleased" , "to be attracted to poacher" , "Made to giggle" .)