Unusual not to be well done (4)
I believe the answer is:
'unusual' is the definition.
(rare can mean unusual or odd)
'not to be well done' is the wordplay.
I cannot really understand how this works, but
'be' could be 'are' (I've seen this in another clue) and 'are' is found in the answer.
A single letter 'r' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rare that I've seen before include "having few matches?" , "Choice - lightly cooked - thin" , "Especially good" , "few and far between" , "little done?" .)