She joins me with a light heart (3)

I believe the answer is:

meg

'she' is the definition.

'me with a light heart' is the wordplay.
'with' is a charade indicator (letters next to each other).
'heart' indicates the central letters.
The central letter of 'light' is 'g'.
'me'+'g'='MEG'

'joins' acts as a link.

I've seen this clue in the Evening Standard.

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