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