20 down
Bertie
develops
from
bad
to
worse
(7)
Bertie develops from bad to worse (7)
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The definition is at the beginning of the clue.
How the clue works
Right, so Bertie is the definition — that's Bertie Wooster from the Jeeves stories. Then "develops from bad" is your anagram indicator, telling you to shuffle the letters of 'to worse' to spell out WOOSTER.