Iodine: I'd iodine stored in curious metal (7)
I believe the answer is:
iridium
'metal' is the definition.
(I know that iridium is a type of metal)
'iodine i'd iodine stored in curious' is the wordplay.
'iodine' becomes 'I' (chemical symbol).
'iodine' becomes 'I' (chemical symbol).
'stored in' means one lot of letters goes inside another.
'curious' becomes 'rum' (synonyms).
'id'+'i'='idi'
'idi' going within 'rum' is 'ridium'.
'i'+'ridium'='IRIDIUM'
(Other definitions for iridium that I've seen before include "Heavy brittle metal used in alloys, Ir" , "Silvery-white metal" , "number 77" , "Metallic element resistant to corrosion, atomic no. 77" , "Hard metal element, Ir" .)