Baby one found in farm building (5)
I believe the answer is:
bairn
'baby' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'one found in farm building' is the wordplay.
'one' becomes 'i' (Roman numeral).
'found in' is an insertion indicator.
'farm building' becomes 'barn' (barn is a kind of farm building).
'i' going into 'barn' is 'BAIRN'.
Can you help me to learn more?
(Other definitions for bairn that I've seen before include "Kid" , "Scot. child" , "little one in Yorkshire" , "A Scottish child?" , "Scots child" .)