Clever person after time becomes idiot (9)
I believe the answer is:
birdbrain
'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" .)