Left-over rail used in construction (8)
I believe the answer is:
residual
'left' is the definition.
(I know that left can be written as residual)
'rail used in construction' is the wordplay.
'rail used' anagrams to 'RESIDUAL'.
But, I am not clear how the anagram is indicated.
'over' acts as a link.
Can you help me to learn more?
(Other definitions for residual that I've seen before include "Remaining at the end" , "Left over as a remainder" , "unused" , "Leftover - used rail (anag)" , "Remaining; repeat royalty" .)