Course of ship going round lake (6)
I believe the answer is:
series
'course' is the definition.
(I've seen this before)
'ship going round lake' is the wordplay.
'ship' becomes 'ss'.
'going round' is an insertion indicator.
'lake' becomes 'erie' (Lake Erie is one of the Canadian/US 'Great Lakes'**).
'ss' placed around 'erie' is 'SERIES'.
'of' is the link.
(Other definitions for series that I've seen before include "Number of related things coming one after another" , "Cycle" , "Number of similar things arranged in order" , "Sequence; tone row" , "Number of similar things occurring in order" .)