Muslim, say, not entering second job (10)
I believe the answer is:
monotheist
'muslim say' is the definition.
(Muslims believe in one god)
'not entering second job' is the wordplay.
'entering' means one lot of letters goes inside another.
'second' becomes 'mo' (both are short periods of time).
'job' becomes 'heist' (job can mean a robbery or heist).
'mo'+'heist'='moheist'
'not' placed into 'moheist' is 'MONOTHEIST'.
(Other definitions for monotheist that I've seen before include "Maybe Christian" , "I'm A Believer" , "Christian, for one" , "One who believes" .)