If you want to be watchful, drink less than a quart (5)
I believe the answer is:
alert
'a quart' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'if you want to be watchful drink less than' is the wordplay.
I cannot really understand how this works, but
'drink' could be 'ale' (ale is a kind of drink) and 'ale' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for alert that I've seen before include "Lively" , "Vigilantly attentive" , "sharp-witted" , "Awake, attentive" , "Signal danger" .)