Bill for example that is unusually desirable (8)
I believe the answer is:
eligible
'desirable' is the definition.
(similar in meaning)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for eligible that I've seen before include "Fit to be chosen" , "Flashing lamp" , "Desirable as a spouse" , "Qualified to be chosen, say a bachelor" , "Worthy to be chosen" .)