Fiercely (5,3,4)
I believe the answer is:
tooth and nail
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fiercely</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for tooth and nail that I've seen before include "(Fight) without restraint" , "With all possible vigour" , "Fiercely" , "With the utmost vigour" , "With greatest vigour" .)