Priest swimming in river shows proficiency (9)
I believe the answer is:
expertise
'proficiency' is the definition.
(I've seen this before)
'priest swimming in river' is the wordplay.
'swimming' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'river' becomes 'exe' (I've seen this before).
'priest' anagrammed gives 'pertis'.
'pertis' inserted inside 'exe' is 'EXPERTISE'.
'shows' acts as a link.
(Other definitions for expertise that I've seen before include "Mastery, proficiency" , "Craft" , "Special skill or knowledge" , "Technical skill" , "Adroitness" .)