Most elusive (6)
I believe the answer is:
rarest
'most elusive' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rarest that I've seen before include "Most scarce and probably precious" , "most excellent" , "Sartre (anag) -- most uncommon" , "Most scarce or least common" , "fine to maximum" .)