Post Office in funds — that’s the intention (7)
I believe the answer is:
purpose
'the intention' is the definition.
('purpose' can be a synonym of 'intention')
'post office in funds' is the wordplay.
'post office' becomes 'po'.
'in' indicates putting letters inside.
'funds' becomes 'purse' (I've seen this before).
'po' going inside 'purse' is 'PURPOSE'.
'that's' is the link.
(Other definitions for purpose that I've seen before include "Will - point" , "End" , "Motivation or intention" , "Goal, aim or intent" , "Resolve; reason" .)