Share drink? That’s the thinking (9)
I believe the answer is:
rationale
'the thinking' is the definition.
(rationale is a kind of thinking)
'share drink?' is the wordplay.
'share' becomes 'ration' (a ration is a share of a limited resource).
'drink?' becomes 'ale' (type of beer).
'ration'+'ale'='RATIONALE'
'that's' acts as a link.
(Other definitions for rationale that I've seen before include "Philosophy" , "Underlying principle" , "Logical reasons (for action)" , "This might account for" , "The logical basis of some thing or action" .)