Fiercely (5,3,4)
I believe the answer is:
tooth and nail
'fiercely' is the definition.
(I've seen this before)
This is all the clue.
(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" .)