Warning to have beer on right temperature (5)
I believe the answer is:
alert
'warning' is the definition.
(alert is a kind of warning)
'beer on right temperature' is the wordplay.
'beer' becomes 'ale' (I've seen this before).
'on' means one lot of letters go next to another.
'right' becomes 'r' (common abbreviation).
'temperature' becomes 't' (abbreviation).
'ale'+'r'+'t'='ALERT'
'to have' acts as a link.
(Other definitions for alert that I've seen before include "Thinking and acting quickly" , "Quick to notice" , "Watchful and observant" , "'Attentive, vigilant (5)'" , "'Attentive, watchful (5)'" .)