Restrict beer, that's the underlying principle (9)
I believe the answer is:
rationale
'the underlying principle' is the definition.
(I've seen this before)
'restrict beer' is the wordplay.
'restrict' becomes 'ration' (rationing is a kind of restricting).
'beer' becomes 'ale' (I've seen this before).
'ration'+'ale'='RATIONALE'
'that's' acts as a link.
(Other definitions for rationale that I've seen before include "solution in theory" , "Underlying principle" , "This might account for" , "Good reason" , "Logical reason for action" .)