Share beer - that shows logic (9)
I believe the answer is:
rationale
'logic' is the definition.
(I've seen this before)
'share beer' is the wordplay.
'share' becomes 'ration' (a ration is a share of a limited resource).
'beer' becomes 'ale' (type of beer).
'ration'+'ale'='RATIONALE'
'that shows' acts as a link.
(Other definitions for rationale that I've seen before include "Logical reasons (for action)" , "Philosophy" , "Reasoned exposition" , "explanation" , "Motivation" .)