Finished at the double? That's certainly not rare (8)
I believe the answer is:
overdone
'certainly not rare' is the definition.
I can't tell whether this definition defines the answer.
'finished at the double?' is the wordplay.
'finished' becomes 'over' (over can mean finished or ended).
'at' is a charade indicator (letters next to each other).
'the double?' becomes 'done' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'over'+'done'='OVERDONE'
'that's' is the link.
Can you help me to learn more?
(Other definitions for overdone that I've seen before include "Cooked to excess" , "Cooked too much or carried too far" , "Exaggerated - far from rare" , "Hammed up" , "Cooked too long" .)