Course of a ship touring a great lake (6)
I believe the answer is:
series
'course' is the definition.
(I've seen this before)
'ship touring a great lake' is the wordplay.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'touring' is an insertion indicator (touring can mean going round).
'a great lake' becomes 'erie' (Erie is one of the Great Lakes).
'ss' enclosing 'erie' is 'SERIES'.
'of a' acts as a link.
(Other definitions for series that I've seen before include "Set of anything" , "Number of similar things occurring in order" , "Rows" , "Sequence; succession" , "Set of episodes" .)