Brandy before going off? (8)

I believe the answer is:

nightcap

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'brandy' is the definition.
The answer and definition can be both things that one consumes for nourishment as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for nightcap that I've seen before include "Patching up with drink at bedtime" , "attire" , "Pre-bed drink" , "Head covering" , "A drink before sleep" .)

I've seen this clue in The Sun.

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