Animal primarily ate mice and rats aboard new vessel (9)
I believe the answer is:
catamaran
'vessel' is the definition.
(catamaran is a type of boat)
'animal primarily ate mice and rats aboard new' is the wordplay.
'animal' becomes 'cat' (cats are animals).
'primarily' indicates taking the first letters.
The first letters of 'ate mice and rats aboard new' is 'amaran'.
'cat'+'amaran'='CATAMARAN'
(Other definitions for catamaran that I've seen before include "Boat with two hulls" , "Sailing vessel" , "Twin-hulled sailing boat" , "Twin-hulled vessel" , "raft of logs" .)