Egg on toast's starter -- is eating out, outside (9)
I believe the answer is:
instigate
'egg on' is the definition.
(thesaurus)
'toast's starter is eating out outside' is the wordplay.
'starter' suggests taking the first letters.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'outside' is an insertion indicator.
The first letter of 'toasts' is 't'.
'is'+'eating'='iseating'
'iseating' is an anagram of 'instigae'.
't' put inside 'instigae' is 'INSTIGATE'.
(Other definitions for instigate that I've seen before include "I taste gin to incite or urge" , "Set off or stir up" , "Start" , "Bring about, initiate" , "I get saint to urge or incite" .)