Block new farm building (4)
I believe the answer is:
barn
'farm building' is the definition.
(barn is a kind of farm building)
'block new' is the wordplay.
'block' becomes 'bar' (block can mean to ban or bar).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'bar'+'n'='BARN'
(Other definitions for barn that I've seen before include "spacious accommodation" , "Outlying farm building" , "stable" , "Farm storage building" , "Large farm outbuilding" .)