Heavy metal, one that's strange, I had one inside (7)
I believe the answer is:
iridium
'heavy metal' is the definition.
(I've seen this before)
'one that's strange i had one inside' is the wordplay.
'one' becomes 'I' (Roman numeral).
'that's strange' becomes 'rum' (similar in meaning).
'i had' becomes 'I'd' (contraction).
'one' becomes 'i' (Roman numeral).
'inside' means one lot of letters goes inside another.
'id'+'i'='idi'
'rum' enclosing 'idi' is 'ridium'.
'i'+'ridium'='IRIDIUM'
(Other definitions for iridium that I've seen before include "Hard metal element, Ir" , "Heavy corrosion-resistant metal, used in alloys" , "Metallic element resistant to corrosion, atomic no. 77" , "Corrosion-resistant metal, Ir" , "Heavy brittle metallic element of the platinum group" .)