Instance of former politician overwhelmed by drink (7)
I believe the answer is:
example
'instance' is the definition.
('example' can be a synonym of 'instance')
'former politician overwhelmed by drink' is the wordplay.
'former' becomes 'ex'.
'politician' becomes 'mp'.
'overwhelmed by' indicates putting letters inside (I've seen this in other clues).
'drink' becomes 'ale' (I've seen this before).
'mp' put within 'ale' is 'ample'.
'ex'+'ample'='EXAMPLE'
'of' is the link.
(Other definitions for example that I've seen before include "Thing illustrating a general rule" , "Fact that illustrates a general rule" , "Someone to emulate" , "Punishment" , "Illustration or model" .)