In the morning priest with sermon shows improvement (12)

I believe the answer is:

amelioration

'improvement' is the definition.
(amelioration is a kind of improvement)

'in the morning priest with sermon' is the wordplay.
'in the morning' becomes 'am'.
'priest' becomes 'eli' (I've seen this before).
'with' means one lot of letters go next to another.
'sermon' becomes 'oration' (I've seen this in other clues).
'am'+'eli'+'oration'='AMELIORATION'

'shows' is the link.

(Another definition for amelioration that I've seen is " Improvement".)

I've seen this clue in The Telegraph.

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