Guide the Italian aboard vessel (5)
I believe the answer is:
pilot
'guide' is the definition.
(pilot can mean a navigator)
'the italian aboard vessel' is the wordplay.
'the italian' becomes 'il' ('the' in Italian).
'aboard' is an insertion indicator.
'vessel' becomes 'pot' (type of container).
'il' placed within 'pot' is 'PILOT'.
(Other definitions for pilot that I've seen before include "trial episode" , "Captain of plane or boat" , "Flier of aircraft" , "Steer, test (scheme)" , "Guide, leader" .)