Behold employment for a parasitic insect (5)
I believe the answer is:
louse
'a parasitic insect' is the definition.
(I've seen this before)
'behold employment' is the wordplay.
'behold' becomes 'lo' (archaic exclamation meaning 'look!').
'employment' becomes 'use' (how something is used).
'lo'+'use'='LOUSE'
'for' is the link.
(Other definitions for louse that I've seen before include "Nit" , "Bloodsucking parasitic insect" , "Unpleasant person" , "Contemptible type" , "Common parasite on animals" .)