Clown and I make a mistake on board vessel (7)
I believe the answer is:
pierrot
'clown' is the definition.
(I know that Pierrot is a type of clown)
'i make a mistake on board vessel' is the wordplay.
'make a mistake' becomes 'err' (to err is to make an error).
'on board' means one lot of letters goes inside another (inserted letters are taken on board).
'vessel' becomes 'pot' (type of container).
'i'+'err'='ierr'
'ierr' going within 'pot' is 'PIERROT'.
'and' is the link.
(Other definitions for pierrot that I've seen before include "I report on French pantomime clown" , "Aged entertainer" , "he's white-faced at work" , "In French panto, clown with white face and baggy costume" , "Male clown from French pantomime" .)