Crossword Genius

Stows a pre-mixed drink which has a powerful effect (5,1,5)

Ross

I believe the answer is:

packs a punch

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'has a powerful effect' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'stows a pre-mixed drink' is the wordplay.
'stows' becomes 'packs' (I can't explain this - if you can you should believe this answer much more).
'pre' is a charade indicator (letters next to each other) ('pre' means before).
'mixed drink' becomes 'punch' (punch is a kind of mixed drink).
'packs'+'a'+'punch'='PACKS A PUNCH'

'which' is the link.

Can you help me to learn more?

I've seen this clue in The Irish Times.
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