Capsized vessel understanding submarine threat (6)
I believe the answer is:
kraken
'submarine threat' is the definition.
The definition suggests a singular noun which matches the answer.
'capsized vessel understanding' is the wordplay.
'capsized' shows that the letters should be reversed in order (in a down clue, the word turns upside down).
'vessel' becomes 'ark' (ark is a kind of vessel).
'understanding' becomes 'ken' (ken can mean someone's range of knowledge).
'ark' backwards is 'kra'.
'kra'+'ken'='KRAKEN'
Can you help me to learn more?
(Other definitions for kraken that I've seen before include "A legendary sea monster of Norwegian myth" , "Sea monster that sleepeth (Tennyson)" , "Mythical sea-monster off Norway's coast" , "Legendary monster of Norwegian seas" , "Mythical sea monster (Norway)" .)