Scottish child - one in farm building (5)
I believe the answer is:
bairn
'scottish child' is the definition.
(Scottish term for a child)
'one in farm building' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'farm building' becomes 'barn' (barn is a kind of farm building).
'i' put inside 'barn' is 'BAIRN'.
(Other definitions for bairn that I've seen before include "Scots child" , "A tot of Scotch" , "Kid" , "Wee child" , "A Scottish child?" .)