... reason being to restrict supply of beer (9)
I believe the answer is:
rationale
'reason' is the definition.
(I've seen this before)
'to restrict supply of beer' is the wordplay.
'to restrict' becomes 'ration' (rationing is a kind of restricting).
'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'
'being' is the link.
Can you help me to learn more?
(Other definitions for rationale that I've seen before include "Why regulate" , "Logical basis (for a course of action)" , "Logical reason for action" , "solution in theory" , "Reasoned exposition" .)