Justification to restrict beer (9)
I believe the answer is:
rationale
'justification' is the definition.
(I've seen this in another clue)
'restrict beer' is the wordplay.
'restrict' becomes 'ration' (rationing is a kind of restricting).
'beer' becomes 'ale' (type of beer).
'ration'+'ale'='RATIONALE'
'to' acts as a link.
(Other definitions for rationale that I've seen before include "Logical reasons (for action)" , "Underlying principle" , "Philosophy" , "Motivation" , "Reasoned exposition" .)