Child, one entering farm building (5)
I believe the answer is:
bairn
'child' is the definition.
(bairn is a kind of child)
'one entering farm building' is the wordplay.
'one' becomes 'i' (Roman numeral).
'entering' is an insertion indicator.
'farm building' becomes 'barn' (barn is a kind of farm building**).
'i' placed into 'barn' is 'BAIRN'.
(Other definitions for bairn that I've seen before include "A tot of Scotch" , "Northern child" , "Kid" , "Scots child" , "Scot. child" .)