It's of all-round interest to mushroom gatherers (5,4)
I believe the answer is:
fairy ring
'interest to mushroom gatherers' is the definition.
I can't judge whether this defines the answer.
'it's of all-round' is the wordplay.
'it's of all' becomes 'fairy' (I can't justify this - if you can you should give a lot more credence to this answer).
'round' becomes 'ring' (synonyms).
'fairy'+'ring'='FAIRY RING'
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" , "Fungal circle" , "Dark circle" .)