See purpose of parasite (5)
I believe the answer is:
louse
'parasite' is the definition.
(thesaurus)
'see purpose' is the wordplay.
'see' becomes 'lo' (archaic word meaning 'look!').
'purpose' becomes 'use' ('use' can be a synonym of 'purpose').
'lo'+'use'='LOUSE'
'of' is the link.
(Other definitions for louse that I've seen before include "Wingless bloodsucking insect" , "Unpleasant person" , "Nit" , "25 [PARASITE]" , "Wingless parasite" .)