"I'll be seeing you every so often," said nice guy (5)

I believe the answer is:

adieu

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'i'll be seeing you' is the definition.
I can't judge whether this defines the answer.

'every so often said nice guy' is the wordplay.
'every so often' means one should take alternating letters.
'said'+'nice'+'guy'='saidniceguy'
The alternate letters of 'saidniceguy' are 'ADIEU'.

Can you help me to learn more?

(Other definitions for adieu that I've seen before include "Goodbye in France" , "the last word in France" , "Comment on leaving" , "I'm away (in France?)" , "the last word in Paris" .)

I've seen this clue in The Independent.

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