Exclude Newton from farm building (4)
I believe the answer is:
barn
'farm building' is the definition.
(barn is a kind of farm building)
'exclude newton' is the wordplay.
'exclude' becomes 'bar' (synonyms).
'newton' becomes 'N' (symbol for the scientific unit of force).
'bar'+'n'='BARN'
'from' acts as a link.
(Other definitions for barn that I've seen before include "Farm 4 down" , "Farm storehouse" , "Large shed" , "Large farm building" , "Type of dance or owl" .)