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' is an insertion indicator.
'about' indicates an anagram.
'a horse' becomes 'gg' (geegee or GG is a childish term for a horse).
'rule' is an anagram of 'luer'.
'luer' enclosing '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" .)