Popular drink I had lacking flavour (7)
I believe the answer is:
insipid
'lacking flavour' is the definition.
(I've seen this before)
'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' (common contraction).
'in'+'sip'+'id'='INSIPID'
(Other definitions for insipid that I've seen before include "Lacking taste or flavour" , "Colourless" , "Wishy-washy" , "Lacking flavour or banal" , "Bland, flavourless" .)