Ship going round a great lake in a line (6)
I believe the answer is:
series
'a line' is the definition.
(I've seen this before)
'ship going round a great lake' is the wordplay.
'ship' becomes 'ss'.
'going round' is an insertion indicator.
'a great lake' becomes 'erie' (I've seen this before**).
'ss' going around 'erie' is 'SERIES'.
'in' acts as a link.
(Other definitions for series that I've seen before include "Similar things happening one after another" , "Set of linked things" , "Sequence of programmes etc." , "Number of related things coming one after another" , "Rows" .)