Most young sheep protected by conservation organisation (6)
I believe the answer is:
newest
'most young' is the definition.
('new' is associated in meaning with 'young')
'sheep protected by conservation organisation' is the wordplay.
'sheep' becomes 'ewes' (I've seen this before).
'protected by' is an insertion indicator.
'conservation organisation' becomes 'nt' (abbreviation for National Trust).
'ewes' going inside 'nt' is 'NEWEST'.
(Other definitions for newest that I've seen before include "most up-to-date" , "Freshest, most modern" , "Latest novel" , "Most recent or most fashionable" , "most recently released?" .)