A bun got thrown at armed vessel (7)
I believe the answer is:
gunboat
'armed vessel' is the definition.
(I've seen this before)
'a bun got thrown' is the wordplay.
'thrown' indicates an anagram.
'a'+'bun'+'got'='abungot'
'abungot' with letters rearranged gives 'GUNBOAT'.
'at' is the link.
(Other definitions for gunboat that I've seen before include "Small naval ship" , "Small military ship" , "Not a bug (anag)" , "Small armed vessel" , "Coastal warship" .)