Something hooted on top of Ray’s tree (8)
I believe the answer is:
'tree' is the definition.
(type of tree)
'something hooted on top of ray's' is the wordplay.
'something hooted' becomes 'horn' (eg you might hoot a car horn).
'on top of' means one lot of letters go next to another.
'ray' becomes 'beam' (synonyms).
(Other definitions for hornbeam that I've seen before include "Resident in wood perhaps" , "is found in wood" , "heavy plant" , "Timber" , "Tree good for hedging" .)