Most sage in Kew is established (6)
I believe the answer is:
wisest
'most sage' is the definition.
('wise' can be similar in meaning to 'sage')
'in kew is established' is the wordplay.
'in' says the answer is hidden in the clue.
'WISEST' can be found hidden inside 'kew is established'.
(Other definitions for wisest that I've seen before include "Most sage and learned" , "Most sage and knowledgeable" , "Most sensible" , "Most astute" , "Closest to Solomon" .)