A certain lady smiles when leaving MS (3)

I believe the answer is:

bea

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'a certain lady' is the definition.
'Bea' can be an answer for 'lady'. I am not sure about the 'a certain' bit.

'smiles when leaving ms' is the wordplay.
'smiles' becomes 'beams' (beaming is a kind of smiling).
'when leaving' is a deletion indicator.
'beams' with 'ms' taken away is 'BEA'.

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