One new loaf you'd say is natural (6)
I believe the answer is:
inbred
'is natural' is the definition.
(similar in meaning)
'one new loaf you'd' is the wordplay.
'one' becomes 'I' (Roman numeral).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'loaf you'd' becomes 'bred' (I can't justify this - if you can you should give a lot more credence to this answer).
'i'+'n'+'bred'='INBRED'
'say' is the link.
Can you help me to learn more?
(Other definitions for inbred that I've seen before include "Existing from birth, derived from parentage" , "from a restricted gene pool?" , "Congenital - unwisely inter-related" , "Inherent" , "native" .)