Advertisement about Spanish river's bottom (9)
I believe the answer is:
posterior
'bottom' is the definition.
(I know that bottom can be written as posterior)
'advertisement about spanish river's' is the wordplay.
'advertisement' becomes 'poster' (poster is a kind of advertisement).
'about' is an insertion indicator.
'spanish river' becomes 'rio' (I've seen this before).
'poster' placed around 'rio' is 'POSTERIOR'.
(Other definitions for posterior that I've seen before include "Behind" , "Rear" , "later" , "Further back; bottom" , "Stern" .)