A large tree or small plant (6)
I believe the answer is:
alpine
'small plant' is the definition.
'alpine' can be an answer for 'plant' (I've seen this before). I am unsure of the 'small' bit.
'a large tree' is the wordplay.
'a large' becomes 'al' (I've seen this in other clues).
'tree' becomes 'pine' (pine tree is a kind of tree).
'al'+'pine'='ALPINE'
'or' acts as a link.
Can you help me to learn more?
(Other definitions for alpine that I've seen before include "Native growing up in mountainous area" , "To do with great European mountains" , "Relating to high mountains" , "Plant of high mountains" , "Relating to a European mountain range" .)