A certain person therefore, will be with me at lunchtime (7)

I believe the answer is:

someone

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'therefore will be with me at lunchtime' is the definition.
I can't tell whether this defines the answer.

'a certain person' is the wordplay.
'a certain' becomes 'some' (eg some person means a certain person).
'person' becomes 'one' ('one' can mean an unspecified person).
'some'+'one'='SOMEONE'

Can you help me to learn more?

(Other definitions for someone that I've seen before include "Important person" , "A person (maybe of importance)" , "big cheese" , "Unspecified person" , "An unknown person" .)

I've seen this clue in the Evening Standard.

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