Oberon has right to replace Puck at last, but gets fungal growth (5,4)
I believe the answer is:
fairy ring
'fungal growth' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'oberon has right to replace puck at last but' is the wordplay.
I cannot quite understand how this works, but
'right' could be 'r' (abbreviation) and 'r' is located in the answer.
'puck' could be 'i' (where the crossword setter is 'Puck') and 'i' is found within the answer.
'at' could be 'in' (synonymous in some cases - eg at school, in school) and 'in' is found in the answer.
'last' could be 'f' (I've seen this in other clues) and 'f' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'gets' acts as a link.
Can you help me to learn more?
(Other definitions for fairy ring that I've seen before include "Circular phenomenon due to fungus, associated with imaginary beings" , "Dark circle" , "Fungal circle" .)