An element of empty unionism on Ulster sash (7)
I believe the answer is:
niobium
'an element' is the definition.
(niobium is a kind of element)
'empty unionism on ulster sash' is the wordplay.
'empty' says to hollow out the word (remove centre letters).
'on' is a charade indicator (letters next to each other).
'ulster' becomes 'NI' (Northern Ireland).
'sash' becomes 'obi' (type of Japanese sash or belt).
'unionism' with its middle removed is 'um'.
'ni'+'obi'='niobi'
'um' after 'niobi' is 'NIOBIUM'.
'of' acts as a link.
(Other definitions for niobium that I've seen before include "Silver-grey metallic element" , "Silvery-grey metal, Nb" , "Superconductive element" , "Silver-grey metal, Nb" , "Chemical element, Nb" .)