French clown and I go off course in vessel (7)
I believe the answer is:
pierrot
'french clown' is the definition.
(clown in French pantomime)
'i go off course in vessel' is the wordplay.
'go off course' becomes 'err' (err can mean to stray off course).
'in' is an insertion indicator.
'vessel' becomes 'pot' (type of container).
'i'+'err'='ierr'
'ierr' placed within 'pot' is 'PIERROT'.
'and' acts as a link.
(Other definitions for pierrot that I've seen before include "French stage character with white face and costume" , "Period; incantation" , "I report on French pantomime clown" , "Male clown from French pantomime" , "French panto clown with whitened face" .)