Scottish child - one in farm building (5)
I believe the answer is:
bairn
'scottish child' is the definition.
(I've seen this in another clue)
'one in farm building' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'farm building' becomes 'barn' (barn is a kind of farm building).
'i' going within '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?" .)