Child - there's one in a farm building (5)
I believe the answer is:
bairn
'child' is the definition.
(Scottish and northern English term for a child)
'one in a farm building' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'a farm building' becomes 'barn' (barn is a kind of farm building).
'i' put within 'barn' is 'BAIRN'.
'there's' acts as a link.
(Other definitions for bairn that I've seen before include "Northern child" , "little one in Yorkshire" , "Scots child" , "A Scottish child?" , "Child (of the North!)" .)