Dispense with pony on leaving with white horses (6)
I believe the answer is:
choppy
'with white horses' is the definition.
('white horses' can refer to foamy waves)
'dispense with pony on leaving' is the wordplay.
'dispense with' becomes 'chop' (chop can mean to cut or dispense with).
'leaving' is a deletion indicator.
'pony' with 'on' taken out is 'py'.
'chop'+'py'='CHOPPY'
(Other definitions for choppy that I've seen before include "Slightly rough (at sea)" , "(Of seas) rough" , "(Of the sea) a bit rough" , "Jerky" , "Describes a rough sea with small waves" .)