Drink wine in which tannin originally flagged (7)
I believe the answer is:
alerted
'flagged' is the definition.
(I've seen this in another clue)
'drink wine in which tannin originally' is the wordplay.
'drink' becomes 'ale' (type of beer).
'wine' becomes 'red wine' (red is a type of wine).
'in which' is an insertion indicator.
'originally' indicates taking the first letters.
The first letter of 'tannin' is 't'.
'red' going around 't' is 'rted'.
'ale'+'rted'='ALERTED'
(Other definitions for alerted that I've seen before include "Put on guard, forewarned" , "Related about warning notified" , "Given signal" , "Put on guard, raised the alarm" , "Warned, made aware" .)