Firstly (2,5,4)
I believe the answer is:
to begin with
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">firstly</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for to begin with that I've seen before include "In the early days" , "Firstly" .)