Vessel breaking a rule about a horse (6)
I believe the answer is:
lugger
'vessel' is the definition.
(lugger is a kind of vessel)
'breaking a rule about a horse' is the wordplay.
'breaking' means one lot of letters goes inside another.
'about' indicates anagramming the letters.
'a horse' becomes 'gg'.
'rule' with letters rearranged gives 'luer'.
'luer' placed around 'gg' is 'LUGGER'.
(Other definitions for lugger that I've seen before include "Small sailing vessel" , "Small ship with four-cornered sails" , "Small sailing ship" , "Small two or three-masted ship" , "Small working boat" .)