Most novel salamander has keys even (6)
I believe the answer is:
newest
'most novel' is the definition.
('new' can be a synonym of 'novel')
'salamander has keys even' is the wordplay.
'salamander' becomes 'newt' (newt is a kind of salamander).
'has' means one lot of letters goes inside another.
'even' means one should take alternating letters (even-numbered letters are used).
The alternate letters of 'keys' are 'es'.
'newt' going around 'es' is 'NEWEST'.
(Other definitions for newest that I've seen before include "Most recent or most fashionable" , "Most original" , "Freshest, most recent" , "Latest novel" , "most recently released?" .)