A large tree growing in European mountains (6)
I believe the answer is:
alpine
'growing in european mountains' is the definition.
(growing in the Alps)
'a large tree' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'tree' becomes 'pine' (pine tree is a kind of tree).
'a'+'l'+'pine'='ALPINE'
(Other definitions for alpine that I've seen before include "from Germany, Austria etc" , "To do with great European mountains" , "Native growing up in mountainous area" , "Plant of high mountains" , "Relating to high mountains" .)