Porter right on the ball (5)
I believe the answer is:
alert
'on the ball' is the definition.
(on the ball can mean alert or paying attention)
'porter right' is the wordplay.
'porter' becomes 'ale' (type of beer).
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'ale'+'rt'='ALERT'
(Other definitions for alert that I've seen before include "Wakeful, observant" , "Signal danger" , "On the ball" , "Thinking and acting quickly" , "Warning signal" .)