In job, Ella regularly observed Christmas (4)
I believe the answer is:
noel
'christmas' is the definition.
(Noel can mean Christmas)
'in job ella regularly observed' is the wordplay.
'regularly observed' indicates alternate letters (letters selected at regular intervals).
'in'+'job'+'ella'='injobella'
The alternate letters of 'injobella' are 'NOEL'.
(Other definitions for noel that I've seen before include "See 7 Across" , "Christmas, in cards" , "namely 4 [COWARD]" , "- - Coward, playwright" , "A lone French Christmas" .)