Sci-fi character’s close shave, heading off across lake (9)
I believe the answer is:
earthling
'sci-fi character's' is the definition.
(sci-fi term for someone from Earth)
'close shave heading off across lake' is the wordplay.
'close shave' becomes 'near thing' (I've seen this before).
'heading off' means to remove the first letter.
'across' is an insertion indicator.
'lake' becomes 'L' (geographical abbreviation).
'nearthing' with its initial letter taken off is 'earthing'.
'earthing' going around 'l' is 'EARTHLING'.
(Other definitions for earthling that I've seen before include "Man of the world" , "man so to be called from space?" , "Human being (sci-fi)" , "sci-fi character" .)