Latin see's new key (9)
I believe the answer is:
essential
'key' is the definition.
(I've seen this in another clue)
'latin see's new' is the wordplay.
'new' is an anagram indicator.
'latin'+'sees'='latinsees'
'latinsees' with letters rearranged gives 'ESSENTIAL'.
(Other definitions for essential that I've seen before include "absolutely neccessary?" , "'Absolutely necessary, required (9)'" , "Indispensible" , "Crucial" , "as oil might be" .)