Reason to restrict supply of booze (9)
I believe the answer is:
rationale
'reason' is the definition.
(I've seen this before)
'restrict supply of booze' is the wordplay.
'restrict' becomes 'ration' (rationing is a kind of restricting).
'supply of booze' becomes 'ale' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ration'+'ale'='RATIONALE'
'to' is the link.
Can you help me to learn more?
(Other definitions for rationale that I've seen before include "Philosophy" , "Underlying principle" , "grounds?" , "solution in theory" , "Logical reason for action" .)