Gaia has a cool crown (6)
I believe the answer is:
icecap
'gaia' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'a cool crown' is the wordplay.
'a cool' becomes 'ice' (I've seen this in another clue).
'crown' becomes 'cap' (synonyms).
'ice'+'cap'='ICECAP'
'has' is the link.
Can you help me to learn more?
(Other definitions for icecap that I've seen before include "Permanent coating of white on a mountain-top" , "Frozen polar region" , "Cold polar region" , "Feature of polar regions" , "Permanent covering of mountain or Pole" .)