Spurned hollow tree blocking gallery display (5)
I believe the answer is:
tetra
'gallery display' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'spurned hollow tree blocking' is the wordplay.
'spurned hollow' becomes 'tr' (I can't justify this - if you can you should give a lot more credence to this answer).
'tree' becomes 'tea' (type of tree).
'blocking' means one lot of letters goes inside another (to block can mean to block in or contain).
'tr' going into 'tea' is 'TETRA'.
Can you help me to learn more?
(Other definitions for tetra that I've seen before include "rarely seen flower" , "Brightly coloured aquarium fish" , "... Four < , in composition" , "This indicates a group of four" , "With four parts" .)