Wee child, one locked in big building (5)
I believe the answer is:
bairn
'wee child' is the definition.
(bairn is a Scottish dialect term for a child)
'one locked in big building' is the wordplay.
'one' becomes 'I' (Roman numeral).
'locked in' indicates putting letters inside.
'big building' becomes 'barn' (building on a farm).
'i' going inside 'barn' is 'BAIRN'.
(Other definitions for bairn that I've seen before include "A Scottish child?" , "Child (of the North!)" , "Wee child" , "Northern child" , "Kid" .)