How to hurt a river in Spain (6)
I believe the answer is:
sprain
'how to hurt' is the definition.
'a river in spain' is the wordplay.
'a river' becomes 'r' (abbreviation for river).
'in' is an insertion indicator.
'r' put inside 'spain' is 'SPRAIN'.
(Other definitions for sprain that I've seen before include "Slight muscle injury, eg to ankle" , "Wrench, dislocate" , "Twist ligament(s)" , "Muscular injury caused by wrench" , "Strain (eg, ankle)" .)