Change menu and put in scarce chemical (9)
I believe the answer is:
neptunium
'scarce chemical' is the definition.
(I've seen this before)
'change menu and put in' is the wordplay.
'change' is an anagram indicator.
'and' means one lot of letters go next to another.
'put'+'in'='putin'
'menu' put after 'putin' is 'putinmenu'.
'putinmenu' with letters rearranged gives 'NEPTUNIUM'.
(Other definitions for neptunium that I've seen before include "scarce chemical" , "Element named after planet" , "Radioactive metal" .)