Occasionally duel and ally gets implacable (6)
I believe the answer is:
deadly
'implacable' is the definition.
(thesaurus)
'occasionally duel and ally' is the wordplay.
'occasionally' means one should take alternating letters.
'duel'+'and'+'ally'='duelandally'
The alternate letters of 'duelandally' are 'DEADLY'.
'gets' acts as a link.
(Other definitions for deadly that I've seen before include "Lethal, capable of causing fatality" , "sniper aspires to be" , "Lethal, capable of causing fatal injury" , "Lethal or mortal" , "Belladonna" .)