Left diamonds for parasites (4)
I believe the answer is:
lice
'parasites' is the definition.
(lice are parasitic insects)
'left diamonds' is the wordplay.
'left' becomes 'l' (common abbreviation).
'diamonds' becomes 'ice' (slang term for diamonds).
'l'+'ice'='LICE'
'for' acts as a link.
(Other definitions for lice that I've seen before include "Wingless bloodsucking parasites" , "Crawlers" , "Bloodsucking parasitic insect" , "Bloodsucking insects" , "Parasitic insects" .)