Behold employment for parasite (5)
I believe the answer is:
louse
'parasite' is the definition.
(type of parasitic insect)
'behold employment' is the wordplay.
'behold' becomes 'lo' (archaic exclamation meaning 'look!').
'employment' becomes 'use' (how something is used).
'lo'+'use'='LOUSE'
'for' acts as a link.
(Other definitions for louse that I've seen before include "Bloodsucking parasitic insect" , "Unpleasant person" , "Despicable type (of insect?)" , "Human parasite" , "Mean person" .)