See the purpose of a parasite (5)
I believe the answer is:
louse
'a parasite' is the definition.
(type of parasitic insect)
'see the purpose' is the wordplay.
'see' becomes 'lo' (archaic exclamation meaning 'look!').
'the purpose' becomes 'use' (synonyms).
'lo'+'use'='LOUSE'
'of' acts as a link.
(Other definitions for louse that I've seen before include "Nit" , "Mean person" , "Wingless bloodsucker" , "Bloodsucking parasitic insect" , "Wingless parasitic insect" .)