Course of a ship touring a great lake (6)
I believe the answer is:
series
'course' is the definition.
(I've seen this before)
'a ship touring a great lake' is the wordplay.
'a ship' becomes 'ss'.
'touring' means one lot of letters goes inside another (touring can mean going round).
'a great lake' becomes 'erie' (I've seen this before**).
'ss' going around 'erie' is 'SERIES'.
'of' 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" .)