Terribly drab old PM who's had many rôles (4,4)

I believe the answer is:

brad pitt

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'had many roles' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'terribly drab old pm' is the wordplay.
'terribly' indicates an anagram.
'old pm' becomes 'Pitt' (Prime Minister William Pitt the Younger or Elder).
'drab' with letters rearranged gives 'brad'.
'brad'+'pitt'='BRAD PITT'

'who's' is the link.

Can you help me to learn more?

(Another definition for brad pitt that I've seen is " Actor".)

I've seen this clue in The Guardian.

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