Er, turned leaves on a German plant (7)
I believe the answer is:
mullein
'plant' is the definition.
(mullein is a kind of plant)
'er turned leaves on a german' is the wordplay.
'er' becomes 'um' (both are expressions of uncertainty).
'turned' says the letters should be written backwards.
'leaves on' becomes 'll' (I can't justify this - if you can you should give a lot more credence to this answer).
'a german' becomes 'ein' ('a' in German).
'um' written backwards gives 'mu'.
'mu'+'ll'+'ein'='MULLEIN'
Can you help me to learn more?
(Other definitions for mullein that I've seen before include "Plant with spikes of yellow flowers" , "Yellow-flowered plant popularly known as Aaron's rod" , "Plant with woolly leaves" .)