Fiercely sound horn, confronting worker with arrest (5,3,4)
I believe the answer is:
tooth and nail
'fiercely' is the definition.
(I've seen this before)
'sound horn confronting worker with arrest' is the wordplay.
'sound horn' becomes 'toot' (I've seen this before).
'confronting' says to put letters next to each other.
'worker' becomes 'hand' (hand is a kind of worker).
'with' says to put letters next to each other.
'arrest' becomes 'nail' (synonyms).
'toot'+'hand'+'nail'='TOOTH AND NAIL'
(Other definitions for tooth and nail that I've seen before include "Fiercely" , "(Fight) without restraint" , "With the utmost vigour" , "With all possible vigour" , "With greatest vigour" .)