Watchful to get the beer right (5)
I believe the answer is:
alert
'watchful' is the definition.
(similar in meaning)
'get the beer right' is the wordplay.
'get' means one lot of letters go next to another.
'the beer' becomes 'ale' (type of beer).
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'ale'+'rt' is 'ALERT'.
'to' acts as a link.
(Other definitions for alert that I've seen before include "'Attentive, watchful (5)'" , "Watchful, quick-thinking" , "Watchfully awake" , "Watchful and bright" , "Watchful and observant" .)