Farm worker finds nothing in sty (4)
I believe the answer is:
peon
'farm worker' is the definition.
(peon can mean an unskilled farm worker)
'nothing in sty' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'in' is an insertion indicator.
'sty' becomes 'pen' (both can mean an animal enclosure).
'o' going into 'pen' is 'PEON'.
'finds' is the link.
(Other definitions for peon that I've seen before include "Day-labourer" , "A Mexican peasant or labourer" , "Spanish-American farm worker" , "Spanish-American farm labourer" , "lowly farm worker" .)