Look to lead exercise to find parasite (5)
I believe the answer is:
louse
'parasite' is the definition.
(I've seen this in other clues)
'look to lead exercise' is the wordplay.
'look' becomes 'lo' (archaic exclamation meaning 'look!').
'to lead' says to put letters next to each other.
'exercise' becomes 'use' (synonyms).
'lo'+'use'='LOUSE'
'to find' acts as a link.
(Other definitions for louse that I've seen before include "Parasite; nasty person" , "Insect -- nasty character, undeserving of respect" , "Bloodsucking parasitic insect" , "Wingless bloodsucking insect" , "Mean person" .)