Firm in the past briefly occupying island (8)
I believe the answer is:
concrete
'firm' is the definition.
(I've seen this before)
'in the past briefly occupying island' is the wordplay.
'in the past' becomes 'once' (I've seen this before).
'briefly' means to remove the last letter.
'occupying' means one lot of letters goes inside another.
'island' becomes 'crete' (Crete is an example**).
'once' with its last letter taken off is 'onc'.
'onc' put into 'crete' is 'CONCRETE'.
(Other definitions for concrete that I've seen before include "Real; building material" , "Not abstract - construction material" , "Building material is not abstract" , "Not abstract or imaginary" , "that may be reinforced" .)