By the sound of it, seaside tree (5)
I believe the answer is:
beech
'tree' is the definition.
(beech is a kind of tree)
'by the sound of it seaside' is the wordplay.
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'seaside' becomes 'beach' (I've seen this before).
'beach' sounds like 'BEECH'.
(Other definitions for beech that I've seen before include "Deciduous tree with smooth grey bark" , "Common deciduous tree" , "Tree of Fagus genus" , "Timber tree" , "for mast?" .)