Obscure king crossing bar, a hazard at sea (3,4)
I believe the answer is:
fog bank
'a hazard at sea' is the definition.
I can't judge whether this definition defines the answer.
'obscure king crossing bar' is the wordplay.
'obscure' becomes 'fog' (synonyms).
'king' becomes 'K' (chess abbreviation).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'bar' becomes 'ban' (bar can mean to ban or prohibit).
'fog'+'k'='fogk'
'fogk' going around 'ban' is 'FOG-BANK'.
Can you help me to learn more?
(Another definition for fog bank that I've seen is " Dense mass of sea mist".)