Greatly value this fictional island (8)

I believe the answer is:

treasure

I'm a little stuck... Click here to teach me more about this clue!

'greatly value' is the definition.
'treasure' can be an answer for 'value' (synonyms). I am unsure of the 'greatly' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for treasure that I've seen before include "Valued possession, buried perhaps" , "much-loved person" , "See 10" , "Value highly" , "... Island (R.L.Stevenson)" .)

I've seen this clue in The Guardian.

Want a hint initially instead of a full solution? Install my app