Unusual not to be well done (4)
I believe the answer is:
rare
'unusual' is the definition.
(rare can mean unusual or odd)
'not to be well done' is the wordplay.
I cannot quite understand how this works, but
'be' could be 'are' (I've seen this in another clue) and 'are' is found within the answer.
A single letter 'r' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
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?" .)