One who saw European tree — it's next to a loch (10)
I believe the answer is:
eyewitness
'one who saw' is the definition.
(someone who saw something happen)
'european tree it's next to a loch' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'tree' becomes 'yew' (type of tree).
'next to' says to put letters next to each other.
'a loch' becomes 'ness' (Loch Ness in Scotland).
'e'+'yew'+'it'+'ness'='EYEWITNESS'
(Other definitions for eyewitness that I've seen before include "He observed" , "One who sees what happens" , "Spectator" , "Onlooker" , "The Observer" .)