Tree by the endless estuary (5)
I believe the answer is:
firth
'estuary' is the definition.
(I know that firth is a type of estuary)
'tree by the endless' is the wordplay.
'tree' becomes 'fir' (fir is a kind of tree).
'by' is a charade indicator (letters next to each other).
'endless' means to remove the last letter.
'the' with its last letter taken off is 'th'.
'fir'+'th'='FIRTH'
(Other definitions for firth that I've seen before include "Scottish sea-inlet" , "Narrow sea inlet in Scotland" , "Estuary" , "Scottish river mouth" , "Arm of the sea - actor Colin" .)