Ferret cut farmer's production in field (4,4)
I believe the answer is:
root crop
'farmer's production in field' is the definition.
(crop whose roots are eaten)
'ferret cut' is the wordplay.
'ferret' becomes 'root' (I am not sure about this - if you are sure you should believe this answer much more).
'cut' becomes 'crop' (cropping is a kind of cutting).
'root'+'crop'='ROOT CROP'
Can you help me to learn more?
(Other definitions for root crop that I've seen before include "Tuber, beet, etc" , "Eg, potatoes" , "Potatoes or carrots" , "Maybe Murphy" , "Swedes, say" .)