Farm building, apart from narrow opening (4)
I believe the answer is:
barn
'farm building' is the definition.
(barn is a kind of farm building)
'apart from narrow opening' is the wordplay.
'apart from' becomes 'bar' (I've seen this in other clues).
'opening' suggests taking the first letters.
The first letter of 'narrow' is 'n'.
'bar'+'n'='BARN'
(Other definitions for barn that I've seen before include "spacious accommodation" , "Large shed" , "Big building" , "Farm 4 down" , "Large farm building" .)