Hunter working either side of river in Spain (5)
I believe the answer is:
orion
'hunter' is the definition.
(constellation)
'working either side of river in spain' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'either side of' means one lot of letters goes inside another (one word goes around another one).
'river in spain' becomes 'rio' (I've seen this in another clue).
'on' enclosing 'rio' is 'ORION'.
(Other definitions for orion that I've seen before include "He hunted" , "Constellation with seven bright stars" , "Large constellation" , "Major constellation" , "Constellation that includes Betelgeuse" .)