Popular drink I had is flat (7)
I believe the answer is:
'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).
'is' acts as a link.
(Other definitions for insipid that I've seen before include "devoid of flavour" , "flat" , "Colourless" , "Wishy-washy" , "Tasteless" .)