Course ship's taken across lake (6)
I believe the answer is:
series
'course' is the definition.
(I've seen this before)
'ship's taken across lake' is the wordplay.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'taken across' indicates putting letters inside.
'lake' becomes 'erie' (Lake Erie is one of the Canadian/US 'Great Lakes').
'ss' enclosing 'erie' is 'SERIES'.
(Other definitions for series that I've seen before include "Number of related things coming one after another" , "A run of related TV or radio programmes" , "Sequence; tone row" , "Similar things happening one after another" , "See 8" .)