Almost unobtainable (4)
I believe the answer is:
rare
'almost unobtainable' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rare that I've seen before include "Might a four-leaf clover be so" , "Seldom - unusual" , "Unusual(ly fine)" , "Very scarce and probably valuable" , "Unusual - exquisite" .)