A fur in the final royal treasure trove (6)
I believe the answer is:
ermine
'a fur' is the definition.
(I know that ermine is a type of fur)
'the final royal treasure trove' is the wordplay.
'final' suggests the final letters.
'royal' becomes 'r' (abbreviation).
'treasure trove' becomes 'mine' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The final letter of 'the' is 'e'.
'e'+'r'+'mine'='ERMINE'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for ermine that I've seen before include "Stoat in its winter coat" , "Expensive white fur" , "In mere strange fur" , "special trimmings" , "Stoat's winter fur" .)