Warning! Beer at room temperature, primarily (5)
I believe the answer is:
alert
'warning' is the definition.
(alert is a kind of warning)
'beer at room temperature primarily' is the wordplay.
'beer' becomes 'ale' (type of beer).
'at' is a charade indicator (letters next to each other).
'primarily' says to take the initial letters.
The initial letters of 'room temperature' is 'rt'.
'ale'+'rt'='ALERT'
(Other definitions for alert that I've seen before include "Watchful and bright" , "Watchful and observant" , "Vigilantly attentive" , "Lively" , "Thinking and acting quickly" .)