Instrument ray finds timber source (8)
I believe the answer is:
hornbeam
'timber source' is the definition.
(hornbeam is a kind of tree)
'instrument ray' is the wordplay.
'instrument' becomes 'horn' (horn is a kind of instrument).
'ray' becomes 'beam' (synonyms).
'horn'+'beam'='HORNBEAM'
'finds' acts as a link.
(Other definitions for hornbeam that I've seen before include "Timber" , "heavy plant" , "Do men abhor this tree?" , "Resident in wood perhaps" , "Tree good for hedging" .)