Sour fruit used in cocktails (6,5)

I believe the answer is:

bitter lemon

I'm a little stuck... Click here to teach me more about this clue!

'sour fruit used in cocktails' is the definition.
I can't tell whether this defines the answer.

'sour fruit' is the wordplay.
'sour' becomes 'bitter' (I've seen this before).
'fruit' becomes 'lemon' (lemon is a kind of fruit).
'bitter'+'lemon'='BITTER LEMON'

Can you help me to learn more?

(Other definitions for bitter lemon that I've seen before include "Soft citrus drink" , "Tart soft drink" .)

I've seen this clue in The Mirror.

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