Be careful with drink — that's the thinking! (9)
I believe the answer is:
rationale
'the thinking' is the definition.
(rationale is a kind of thinking)
'be careful with drink' is the wordplay.
'be careful with' becomes 'ration' (to ration a supply is to be careful with it).
'drink' becomes 'ale' (ale is a kind of drink).
'ration'+'ale'='RATIONALE'
'that's' is the link.
(Other definitions for rationale that I've seen before include "Logical basis (for a course of action)" , "This might account for" , "Reasoned exposition" , "Philosophy" , "solution in theory" .)