Protocol largely covering arrest (6)
I believe the answer is:
collar
'arrest' is the definition.
(synonyms)
'protocol largely covering' is the wordplay.
'covering' indicates the answer is hidden within the clue.
'COLLAR' is hidden in the letters of 'protocol largely'.
(Other definitions for collar that I've seen before include "Grab one" , "Be hot under the . . . . . . to be angry or resentful" , "Appropriate - piece of neckwear" , "Seize; cut of bacon" , "A cloth neck-band" .)