A small problem with Queen impersonator? (7)

I believe the answer is:

assumer

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'impersonator?' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'a small problem with queen' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'problem' becomes 'sum' (arithmetic problem).
'with' means one lot of letters go next to another.
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'a'+'s'+'sum'+'er'='ASSUMER'

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I've seen this clue in The Times and The Sunday Times.

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