Motivation to share drink (9)
I believe the answer is:
rationale
'motivation' is the definition.
(I've seen this before)
'share drink' is the wordplay.
'share' becomes 'ration' (ration is a kind of share).
'drink' becomes 'ale' (I've seen this before).
'ration'+'ale'='RATIONALE'
'to' is the link.
(Other definitions for rationale that I've seen before include "Good reason" , "Why regulate" , "This might account for" , "grounds?" , "Reasoned exposition" .)