It's frightening crossing lake -- cover needed (7)
I believe the answer is:
cowling
'cover needed' is the definition.
'it's frightening crossing lake' is the wordplay.
'it's frightening' becomes 'cowing'.
'crossing' indicates putting letters inside.
'lake' becomes 'l' (geographical abbreviation).
'cowing' placed around 'l' is 'COWLING'.
(Other definitions for cowling that I've seen before include "aeroplane's engine casing" , "Removable engine cover" .)