What the farmer needs is a new silo (4)
I believe the answer is:
soil
'what the farmer needs' is the definition.
I can't tell whether this defines the answer.
'new silo' is the wordplay.
'new' is an anagram indicator.
'silo' with letters rearranged gives 'SOIL'.
'is a' is the link.
Can you help me to learn more?
(Other definitions for soil that I've seen before include "Top layer of the surface of the earth" , "Earth for growing" , "Dirty - dirt" , "Earth; stain" , "Loose upper layer of earth" .)