Heard why seaweed from the East is rich in essential element (5)
I believe the answer is:
irony
'rich in essential element' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'heard why seaweed from the east' is the wordplay.
'heard why' becomes 'y' (I am not sure about the 'heard' bit.).
'seaweed' becomes 'nori' (I've seen this before).
'from the east' shows that the letters should be reversed in order (on a map from right to left).
'y'+'nori'='ynori'
'ynori' backwards is 'IRONY'.
'is' is the link.
Can you help me to learn more?
(Other definitions for irony that I've seen before include "Satirical speech form" , "Caustic remark" , "Use of words to imply the opposite of what they mean" , "like a metallic element" , "the reverse of what you'd expect" .)