Hair treatment provided by barber in Seville (5)
I believe the answer is:
rinse
'hair treatment' is the definition.
(I've seen this in another clue)
'provided by barber in seville' is the wordplay.
'provided by' indicates the answer is hidden within the clue (the answer is provided within the clue).
'RINSE' is hidden in the letters of 'barber in seville'.
(Other definitions for rinse that I've seen before include "Flush" , "blue one on top?" , "Remove soap with clean water" , "Mouth cleansing solution" , "Wash - siren (anag)" .)