Libertine reluctant to go on a Spanish river (8)
I believe the answer is:
lothario
'libertine' is the definition.
(Lothario is an example)
'reluctant to go on a spanish river' is the wordplay.
'reluctant' becomes 'loth' ('loth' can be a synonym of 'reluctant').
'to go on' says to put letters next to each other (in a down clue, some letters go on top of others).
'spanish river' becomes 'rio' (I've seen this before).
'loth'+'a'+'rio'='LOTHARIO'
(Other definitions for lothario that I've seen before include "Gay seducer" , "Heartbreaker" , "Don Juan-type character" , "Libertine - ol' hot air (anag)" , "Womaniser, philanderer" .)