Crossword Genius

Late drink that may go to the head (8)

Ross

I believe the answer is:

nightcap

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'late drink that may go to the head' is the definition.
'nightcap' can be an answer for 'late drink' (drink drunk late at night). I am not certain of the rest of the definition.

'late drink' is the wordplay.
'late' becomes 'night' (I've seen this before).
'drink' becomes 'cap' (I can't justify this - if you can you should believe this answer much more).
'night'+'cap'='NIGHTCAP'

Can you help me to learn more?

(Other definitions for nightcap that I've seen before include "Old headgear" , "Last drink of the day?" , "that may go to one's head?" , "drink having sleepy effect?" , "A drink before sleep" .)

I've seen this clue in The Telegraph.
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