Difficult crossing area with unknown danger (6)
I believe the answer is:
hazard
'danger' is the definition.
(I know that hazard is a type of danger)
'difficult crossing area with unknown' is the wordplay.
'difficult' becomes 'hard' (synonyms).
'crossing' is an insertion indicator.
'area' becomes 'a' (maths abbreviation).
'with' says to put letters next to each other.
'unknown' becomes 'z' (algebra).
'a' after 'z' is 'za'.
'hard' going around 'za' is 'HAZARD'.
(Other definitions for hazard that I've seen before include "Endanger, jeopardize" , "Golf course obstacle" , "Danger; gamble" , "Obstacle, source of danger" , "Risk; guess" .)