Motivation to share beer (9)
I believe the answer is:
rationale
'motivation' 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'
'to' is the link.
(Other definitions for rationale that I've seen before include "explanation" , "Motivation" , "Basis of logic" , "This might account for" , "Good reason" .)