Zombie oath used regularly in Caribbean sorcery (5)
I believe the answer is:
obeah
'caribbean sorcery' is the definition.
(I've seen this in another clue)
'zombie oath used regularly' is the wordplay.
'used regularly' indicates alternate letters.
'zombie'+'oath'='zombieoath'
The alternating letters of 'zombieoath' are 'OBEAH'.
'in' acts as a link.
(Other definitions for obeah that I've seen before include "Caribbean witchcraft" , "that's charming" , "amulet" , "West Indian witchcraft" , "African sorcery" .)