Plant containing source of metallic element (9)
I believe the answer is:
germanium
'metallic element' is the definition.
'germanium' can be an answer for 'element' (germanium is a kind of element). I am unsure of the 'metallic' bit.
'plant containing source of' is the wordplay.
'plant' becomes 'geranium' (geranium is a plant).
'containing' indicates putting letters inside.
'source of' becomes 'm' (I can't justify this - if you can you should give a lot more credence to this answer).
'geranium' going around 'm' is 'GERMANIUM'.
(Other definitions for germanium that I've seen before include "Semiconducting element" , "semiconductor material" .)