Marathon taking place, cutting through Lake Geneva, initially (4)
I believe the answer is:
long
'marathon' is the definition.
(marathon can mean long or lengthy)
'taking place cutting through lake geneva initially' is the wordplay.
'taking place' becomes 'on' (as in a phrase like 'the event is on now').
'cutting through' means one lot of letters goes inside another.
'initially' says to take the initial letters.
The first letters of 'lake geneva' is 'lg'.
'on' going within 'lg' is 'LONG'.
(Other definitions for long that I've seen before include "to be tall" , "Extensive - pine" , "hunger" , "dream" , "Thus having been treated by stretcher, perhaps" .)