Johnson crooked, slightly lacking, a whimsical character behind bars? (5,5)

I believe the answer is:

willy wonka

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'whimsical character behind bars?' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'johnson crooked slightly lacking a' is the wordplay.
'johnson' becomes 'willy' (I can't explain this - if you can you should give a lot more credence to this answer).
'crooked' becomes 'wonky' (similar in meaning).
'slightly lacking' means to remove the last letter.
'wonky' with its last letter taken off is 'wonk'.
'willy'+'wonk'+'a'='WILLY WONKA'

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I've seen this clue in The Guardian.

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