To begin with, mince tarts! (5)
I believe the answer is:
start
'to begin' is the definition.
('start' can be a synonym of 'begin')
'mince tarts' is the wordplay.
'mince' indicates an anagram.
'tarts' anagrammed gives 'START'.
'with' is the link.
(Other definitions for start that I've seen before include "Outset" , "Origin" , "Opening - jump" , "Begin to recoil?" , "Commencement" .)