Strange enveloping layer is half metal (7)
I believe the answer is:
rhenium
'metal' is the definition.
(I know that rhenium is a type of metal)
'strange enveloping layer is half' is the wordplay.
'strange' becomes 'rum' (synonyms).
'enveloping' means one lot of letters goes inside another.
'layer' becomes 'hen' (hens lay eggs).
'half' suggests halving the letters.
'is' halved is 'i'.
'hen'+'i'='heni'
'rum' going around 'heni' is 'RHENIUM'.
(Other definitions for rhenium that I've seen before include "Chemical" , "Element (Re)" , "Metal" .)