Like a blue lobster (4)
I believe the answer is:
rare
'like a blue lobster' 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 "Very scarce and probably valuable" , "Ben Jonson was considered so" , "early" , "Singular, infrequent" , "Exceptional" .)