Navigation hazard (4)
I believe the answer is:
berg
'navigation hazard' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for berg that I've seen before include "Mountain; shipping hazard" , "feature of icy waters" , "One who scored" , "Alban ----, composer of the incomplete opera 'Lulu' (1935)" , "Large mass of floating ice" .)