Perhaps Merkel's eaten 40% of icing that's used on cakes (8)
I believe the answer is:
angelica
'that's used on cakes' is the definition.
(plant with green stalks)
'perhaps merkel's eaten 40%' is the wordplay.
'perhaps merkel' becomes 'Angela' (Angela Merkel, Chancellor of Germany).
'eaten' is an insertion indicator (some letters have eaten others).
'40%' becomes 'ic' (I am not sure about this - if you are sure you should believe this answer much more).
'angela' going around 'ic' is 'ANGELICA'.
'of icing' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for angelica that I've seen before include "Candied plant stalks" , "Candied leaf-stalks, used to decorate cakes" , "Candied stem, decorates cake" , "Nag Alice to get fragrant plant" , "Its candied leaves are used in cake decoration" .)