Hunter about to cross Spanish river (5)
I believe the answer is:
orion
'hunter' is the definition.
(constellation)
'about to cross spanish river' is the wordplay.
'about' becomes 'on' (as in a book 'on' a certain topic).
'to cross' means one lot of letters goes inside another.
'spanish river' becomes 'rio' (I've seen this in another clue).
'on' placed around 'rio' is 'ORION'.
(Other definitions for orion that I've seen before include "Mythical Greek hunter turned into constellation" , "Star" , "Group of stars" , "Bright constellation, named after legendary Greek hunter" , "Greek hunter became a constellation" .)