Suppose the metal is inside (8)
I believe the answer is:
theorise
'suppose' is the definition.
(I know that suppose can be written as theorise)
'the metal is inside' is the wordplay.
'metal' becomes 'ore' (I have seen 'Raw metal ' mean 'ore' so perhaps 'metal' could also mean 'ore').
'inside' means one lot of letters goes inside another.
'ore' going around 'is' is 'orise'.
'the'+'orise'='THEORISE'
(Other definitions for theorise that I've seen before include "Express ideas" , "Form of hypothesis or conjecture" , "Form view" , "Conjecture, hypothesise" , "Think up possible explanations" .)