Popular drink I had in flat (7)
I believe the answer is:
insipid
'flat' is the definition.
('insipid' can be a synonym of 'flat')
'popular drink i had' is the wordplay.
'popular' becomes 'in'.
'drink' becomes 'sip' (I've seen this before).
'i had' becomes 'id' (I'd).
'in'+'sip'+'id'='INSIPID'
'in' acts as a link.
(Other definitions for insipid that I've seen before include "Wishy-washy" , "Jejune" , "Boring and lacking liveliness" , "Tasteless" , "Bland, flavourless" .)