In the main its chief danger remains hidden (7)
I believe the answer is:
iceberg
'in' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for iceberg that I've seen before include "that has hidden depths?" , "Floating mass (of lettuce?)" , "Floating shipping hazard" , "Largely hidden danger" , "Type of 16 Down" .)