Whip the Spanish horse (4)
I believe the answer is:
lash
'whip' is the definition.
(I know that whip can be written as lash)
'the spanish horse' is the wordplay.
'the spanish' becomes 'las' ('the' in Spanish).
'horse' becomes 'H' (abbreviation as in horse power).
'las'+'h'='LASH'
(Other definitions for lash that I've seen before include "Whip with a sharp blow" , "Flog" , "Strike with a whip" , "Whip or scourge" , "Stroke of the whip" .)