Succeed effortlessly by a lake at the seaside (7)
I believe the answer is:
coastal
'the seaside' is the definition.
(I've seen this in another clue)
'succeed effortlessly by a lake' is the wordplay.
'succeed effortlessly' becomes 'coast' (I've seen this in another clue).
'by' means one lot of letters go next to another.
'lake' becomes 'L' (geographical abbreviation).
'coast'+'a'+'l'='COASTAL'
'at' is the link.
(Other definitions for coastal that I've seen before include "Of the seashore" , "Littoral" , "of shoreline" , "seaboards" , "Near the sea" .)