Shut up about old farm worker (4)
I believe the answer is:
peon
'farm worker' is the definition.
(peon can mean an unskilled farm worker)
'shut up about old' is the wordplay.
'shut up' becomes 'pen' (pen can mean to shut in or enclose).
'about' is an insertion indicator.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'pen' going around 'o' is 'PEON'.
(Other definitions for peon that I've seen before include "Day-labourer" , "Latin American farm labourer" , "Spanish-American farm labourer" , "A Mexican peasant or labourer" , "lowly farm worker" .)