One disloyal in a job, source of trouble (8)
I believe the answer is:
apostate
'one disloyal' is the definition.
(thesaurus)
'a job source of trouble' is the wordplay.
'job' becomes 'post' (post is a kind of job).
'source of trouble' becomes 'ate' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'post'+'ate'='APOSTATE'
'in' is the link.
Can you help me to learn more?
(Other definitions for apostate that I've seen before include "Renegade" , "Person who deserts their religion" , "Julian was such" , "Religious renouncer" , "Someone who has abandoned their religion" .)