Up to work the fields (4)
I believe the answer is:
till
This could be a double definition.
'up to' is the first definition.
(up to or till a certain time)
'work the fields' is the second definition.
'till' can be an answer for 'work' (to till is to work the land). I'm unsure of the 'the fields' bit.
Can you help me to learn more?
(Other definitions for till that I've seen before include "Small box in shop for holding cash" , "Cash machine" , "Farm - money drawer" , "Cash found here" , "not starting yet" .)