Having finished dinner first, one exaggerated (8)
I believe the answer is:
overdone
'one exaggerated' is the definition.
(I've seen this before)
'having finished dinner first' is the wordplay.
I cannot really see how this works, but
'finished' could be 'over' (over can mean finished or ended) and 'over' is found in the answer.
'first' could be 'one' (I've seen this in another clue) and 'one' is present in the answer.
A single letter 'd' 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 overdone that I've seen before include "really a bit much!" , "Hammed up" , "Cooked to excess" , "Exaggerated; cooked too much" , "Cooked too much or carried too far" .)