One is kept in farm buildings for kids (6)
I believe the answer is:
bairns
'kids' is the definition.
(bairn is a kind of kid)
'one is kept in farm buildings' is the wordplay.
'one' becomes 'i' (Roman numeral).
'is kept in' means one lot of letters goes inside another.
'farm buildings' becomes 'barns' (barn is a kind of farm building).
'i' put inside 'barns' is 'BAIRNS'.
'for' acts as a link.
(Other definitions for bairns that I've seen before include "Scottish children with brains" , "Children, in Scotland" , "Scottish wee ones" .)