Most scarce (6)
I believe the answer is:
rarest
'most scarce' is the definition.
('rare' can be similar in meaning to 'scarce')
This is the entire clue.
(Other definitions for rarest that I've seen before include "Sartre (anag) -- most uncommon" , "Least common, most unusual" , "most excellent" , "fine to maximum" , "Most scarce or infrequent" .)