Limit supply of beer in grounds (9)
I believe the answer is:
rationale
'grounds' is the definition.
(I've seen this before)
'limit supply of beer' is the wordplay.
'limit' becomes 'ration' (rationing is a kind of limiting).
'supply of beer' becomes 'ale' (I can't explain this - if you can you should give a lot more credence to this answer).
'ration'+'ale'='RATIONALE'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for rationale that I've seen before include "Basis of logic" , "Underlying principle" , "justification" , "explanation" , "Philosophy" .)