Metal miners surrounding old boy among islands (7)
I believe the answer is:
niobium
'metal' is the definition.
(metallic element)
'miners surrounding old boy among islands' is the wordplay.
'miners' becomes 'NUM' (National Union of Mineworkers).
'surrounding' is an insertion indicator.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'among' means one lot of letters goes inside another (inserted letters are placed among others).
'islands' becomes 'ii' (an abbreviation for island written twice).
'ob' inserted into 'ii' is 'iobi'.
'num' placed around 'iobi' is 'NIOBIUM'.
(Other definitions for niobium that I've seen before include "Chemical element, Nb" , "Silver-grey metal, Nb" , "Silver-grey metallic element" , "Superconductive element" , "Silvery-grey metal, Nb" .)