Reason to conserve beer (9)
I believe the answer is:
rationale
'reason' is the definition.
(I've seen this before)
'conserve beer' is the wordplay.
'conserve' becomes 'ration' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'beer' becomes 'ale' (type of beer).
'ration'+'ale'='RATIONALE'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for rationale that I've seen before include "Basis of logic" , "solution in theory" , "This might account for" , "Why regulate" , "Logical reasons (for action)" .)