Crossword Genius

Popular drink I had is flat (7)

Ross

I believe the answer is:

insipid

'flat' is the definition.
(I know that flat can be written as insipid)

'popular drink i had' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'drink' becomes 'sip' (sipping is a kind of drinking).
'i had' becomes 'I'd' (contraction).
'in'+'sip'+'id'='INSIPID'

'is' acts as a link.

(Other definitions for insipid that I've seen before include "devoid of flavour" , "flat" , "Colourless" , "Wishy-washy" , "Tasteless" .)

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