Reasons to control allowance of beer (9)
I believe the answer is:
rationale
'reasons' is the definition.
(I've seen this before)
'control allowance of beer' is the wordplay.
'control allowance of' becomes 'ration' (to ration something is to control the supply of it).
'beer' becomes 'ale' (I've seen this before).
'ration'+'ale'='RATIONALE'
'to' is the link.
(Other definitions for rationale that I've seen before include "solution in theory" , "Motivation" , "explanation" , "Philosophy" , "Good reason" .)