Tree, by the sound of it, that's found at the seaside (5)
I believe the answer is:
beach
'the seaside' is the definition.
(a beach is found by the seaside)
'tree by the sound of it' is the wordplay.
'tree' becomes 'beech' (beech tree is a kind of tree).
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'beech' sounds like 'BEACH'.
'that's found at' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for beach that I've seen before include "The shore between high and low water-mark" , "Accessible shoreline" , "Area of land sloping down to water" , "Pebbly or sandy shore" , "Stretch of coastline" .)