Priest to have gloomy expression (5)
I believe the answer is:
frown
'gloomy expression' is the definition.
(I've seen this before)
'priest to have' is the wordplay.
'priest' becomes 'fr'.
'to have' becomes 'own' ('own' can be a synonym of 'have').
'fr'+'own'='FROWN'
(Other definitions for frown that I've seen before include "Indicate disapproval" , "Wrinkle brow" , "Furrow one's brow" , "Show disapproval" , "Look worried" .)