Crossword Genius

Clever person after time becomes idiot (9)

Ross

I believe the answer is:

birdbrain

I'm a little stuck... Click here to teach me more about this clue!

'idiot' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe you can see a link between them that I don't see?

'clever person after time' is the wordplay.
'clever person' becomes 'brain' (I've seen this before).
'after' is a charade indicator (letters next to each other).
'time' becomes 'bird' (both are slang for a prison sentence).
'brain' after 'bird' is 'BIRDBRAIN'.

'becomes' is the link.

Can you help me to learn more?

(Other definitions for birdbrain that I've seen before include "Twittering twit?" , "Fool" , "Silly person (with sparrow's mind?)" , "Stupid" .)

I've seen this clue in The Telegraph.
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