Initial success as gnat, firstly, squished (6,5)
I believe the answer is:
flying start
'initial success' is the definition.
(getting off to a flying start is starting well)
'gnat firstly squished' is the wordplay.
'squished' indicates an anagram.
'gnat'+'firstly'='gnatfirstly'
'gnatfirstly' is an anagram of 'FLYING START'.
'as' is the link.
(Other definitions for flying start that I've seen before include "Advantageous beginning" , "See 6" .)