One leaving Scots child in outhouse (4)
I believe the answer is:
barn
'outhouse' is the definition.
(I've seen this before)
'one leaving scots child' is the wordplay.
'one' becomes 'I' (Roman numeral).
'leaving' indicates named letters should be taken away.
'scots child' becomes 'bairn' (I've seen this before).
'bairn' with 'i' taken away is 'BARN'.
'in' acts as a link.
(Other definitions for barn that I've seen before include "Farm building for storing hay or grain" , "Type of dance or owl" , "Large outhouse" , "Outlying farm building" , "Large shed" .)