Agonising undertaking for farm workers! (9)
I believe the answer is:
harrowing
I believe this is a double definition.
'agonising' is the first definition.
'undertaking for farm workers' is the second definition.
(agricultural process)
(Other definitions for harrowing that I've seen before include "traumatic" , "Breaking up soil or distressing greatly" , "Acutely distressing" , "work for a farmer" , "Breaking new ground" .)