Appeals to a sweetheart to be nice (6)

I believe the answer is:

please

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'be nice' is the definition.
I can't tell whether this definition defines the answer.

'appeals to a sweetheart' is the wordplay.
'appeals' becomes 'pleas' (plea is a kind of appeal).
'to a sweetheart' becomes 'e' (the heart/centre of 'sweet').
'pleas'+'e'='PLEASE'

'to' acts as a link.

Can you help me to learn more?

(Other definitions for please that I've seen before include "Polite word of request" , "Cause (someone) to feel satisfied" , "Be good enough to" , "Gratify, make contented" , "Satisfy while asleep" .)

I've seen this clue in the Evening Standard.

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