Coniferous tree, with rascal on stump (6,6)
I believe the answer is:
monkey puzzle
'coniferous tree' is the definition.
(monkey puzzle is a kind of coniferous tree)
'with rascal on stump' is the wordplay.
'with' means one lot of letters go next to another.
'rascal' becomes 'monkey' (synonyms).
'on stump' becomes 'puzzle' (I am not sure about this - if you are sure you should believe this answer much more).
'monkey' next to 'puzzle' is 'MONKEY PUZZLE'.
Can you help me to learn more?
(Other definitions for monkey puzzle that I've seen before include "Chile pine (the Araucaria tree)" , "CHILEPINE" , "ARAUCARIA to remember" , "Chilean tree" .)