Sauce required for a formal dinner? (8)

I believe the answer is:

dressing

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'sauce' is the definition.
(I know that dressing is a type of sauce)

'formal dinner?' is the wordplay.
'formal' becomes 'dress' (similar in meaning).
'dinner?' becomes 'ing' (I can't explain this - if you can you should give a lot more credence to this answer).
'dress'+'ing'='DRESSING'

'required for a' is the link.

Can you help me to learn more?

(Other definitions for dressing that I've seen before include "Sauce for salad etc" , "plaster, for example?" , "bedroom procedure?" , "Putting clothes on" , "but not stripping!" .)

I've seen this clue in The Globe and Mail.

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