Aquatic bird priest caught boarding vessel (7)
I believe the answer is:
pelican
'aquatic bird' is the definition.
(pelican is a kind of aquatic bird)
'priest caught boarding vessel' is the wordplay.
'priest' becomes 'eli' (biblical figure).
'caught' becomes 'c' (cricket abbreviation).
'boarding' is an insertion indicator.
'vessel' becomes 'pan' (vessel or container for cooking).
'eli'+'c'='elic'
'elic' inserted inside 'pan' is 'PELICAN'.
(Other definitions for pelican that I've seen before include "nickname of US State" , "Tropical bird" , "Seabird having a large bill with a pouch for fish" , "Seabird with a large pouched bill" , "Large waterbird" .)