Warning: wine before beer starts off rumbly tummy (3,5)
I believe the answer is:
red alert
'warning' is the definition.
(I've seen this before)
'wine before beer starts off rumbly tummy' is the wordplay.
'wine' becomes 'red' (red is a type of wine).
'before' is a charade indicator (letters next to each other).
'beer' becomes 'ale' (type of beer).
'starts off' indicates taking the first letters.
The initial letters of 'rumbly tummy' is 'rt'.
'red'+'ale'+'rt'='RED ALERT'
(Other definitions for red alert that I've seen before include "State of readiness for imminent crisis" , "Severe 1 dn" , "Severe warning" , "Urgent warning" , "Alarm" .)