Old priest's upset in clash with prison officer (6)
I believe the answer is:
jailer
'prison officer' is the definition.
(I know that prison officer can be written as jailer)
'old priest's upset in clash' is the wordplay.
'old priest' becomes 'eli' (Biblical character).
'upset' says the letters should be written backwards.
'in' indicates putting letters inside.
'clash' becomes 'jar' (synonyms).
'eli' written backwards gives 'ile'.
'ile' inserted inside 'jar' is 'JAILER'.
'with' is the link.
(Other definitions for jailer that I've seen before include "one professionally deployed with a pen?" , "Old-fashioned screw" , "Sort of guard" , "Prison warder" , "Prison guard" .)