The underlying reason is to share out the beer (9)
I believe the answer is:
rationale
'the underlying reason' is the definition.
(I've seen this in another clue)
'share out the beer' is the wordplay.
'share out' becomes 'ration'.
'the beer' becomes 'ale' (type of beer).
'ration'+'ale'='RATIONALE'
'is to' acts as a link.
(Other definitions for rationale that I've seen before include "Logic, reasoning" , "Logical reason for action" , "This might account for" , "The logical basis of some thing or action" , "Logical reasons (for action)" .)