One imprisoned in lead alloy by English (8)
I believe the answer is:
internee
'one imprisoned' is the definition.
(somebody who has been interned)
'in lead alloy by english' is the wordplay.
'lead alloy' becomes 'terne' (steel covered in a tin and lead alloy).
'by' means one lot of letters go next to another.
'english' becomes 'E' (abbreviation).
'in'+'terne'+'e'='INTERNEE'
(Other definitions for internee that I've seen before include "Suspect held" , "One kept prisoner" , "Prisoner of war living in armed camp or prison" , "One imprisoned during wartime" , "Inmate" .)