...thinking to conserve beer (9)
I believe the answer is:
rationale
'thinking' is the definition.
(rationale is a kind of thinking)
'conserve beer' is the wordplay.
'conserve' becomes 'ration' (I can't explain this - if you can you should believe this answer much more).
'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 "Motivation" , "The logical basis of some thing or action" , "Basis of logic" , "Logic, reasoning" , "Underlying principle" .)