Begin to drink bracing bitter (5,2)
I believe the answer is:
start up
'begin' is the definition.
(I know that start up is a more specific form of begin)
'drink bracing bitter' is the wordplay.
'drink' becomes 'sup' (to sup is to drink).
'bracing' is an insertion indicator (bracing can mean securing or surrounding).
'bitter' becomes 'tart' (tart can mean bitter-tasting).
'sup' enclosing 'tart' is 'START-UP'.
'to' is the link.
(Other definitions for start up that I've seen before include "Newly-formed company" , "newly formed company" , "Initiation" , "fresh concern" , "Newly established business" .)