Surplus rails due to be recycled (8)
I believe the answer is:
residual
'surplus' is the definition.
(I've seen this before)
'rails due to be recycled' is the wordplay.
'to be recycled' is an anagram indicator.
'rails'+'due'='railsdue'
'railsdue' anagrammed gives 'RESIDUAL'.
(Other definitions for residual that I've seen before include "surplus" , "Leftover; repeat royalty" , "Left over as a remainder" , "Pertaining to what is left over" , "'Remaining, left over (8)'" .)