Nuclear shelter is hazard of course (6)
I believe the answer is:
bunker
This might be a double definition.
'nuclear shelter' is the first definition.
'bunker' can be an answer for 'shelter' (I've seen this before). I am not certain of the 'nuclear' bit.
'hazard of course' is the second definition.
'bunker' can be an answer for 'hazard' (bunker is a kind of hazard). I'm not certain of the 'of course' bit.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for bunker that I've seen before include "Belief that one is safe, despite the evidence" , "Underground shelter - golfing hazard" , "Sand trap" , "Underground shelter, store" , "Wartime shelter" .)