Horse in oceangoing vessel fed unsuitable food (10)
I believe the answer is:
lipizzaner
'horse' is the definition.
(I know that Lipizzaner is a horse breed)
'oceangoing vessel fed unsuitable food' is the wordplay.
'oceangoing vessel' becomes 'liner' (ocean liner).
'fed' means one lot of letters goes inside another (one word is 'fed' to another).
'unsuitable food' becomes 'pizza' (type of food).
'liner' enclosing 'pizza' is 'LIPIZZANER'.
'in' is the link.
(Other definitions for lipizzaner that I've seen before include "Grey" , "Breed of horse in the Spanish Riding School of Vienna" .)