Crossword Genius

What pub low on stock may do briefly is prudent (8)

Ross

I believe the answer is:

rational

I'm a little stuck... Click here to teach me more about this clue!

'prudent' is the definition.
(thesaurus)

'what pub low on stock may do briefly' is the wordplay.
I cannot quite understand how this works, but
'on' could be 'at' (I've seen this in other clues) and 'at' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

'is' is the link.

Can you help me to learn more?

(Other definitions for rational that I've seen before include "Sane; able to think" , "Capable of thought" , "Logical; sensible" , "Having common sense or reason" , "Thinking logically" .)

I've seen this clue in The Independent.
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