Ran a horse into the sea (7)
I believe the answer is:
managed
'ran' is the definition.
(I've seen this before)
'a horse into the sea' is the wordplay.
'horse' becomes 'nag' (nag is a kind of horse).
'into' indicates putting letters inside.
'the sea' becomes 'med' (abbreviation of Mediterranean**).
'a'+'nag'='anag'
'anag' put within 'med' is 'MANAGED'.
(Other definitions for managed that I've seen before include "Controlled, was in charge of" , "Supervised (staff)" , "Contrived" , "Ran, administered" , "Coped" .)