Surplus engineers left cars round (8)
I believe the answer is:
residual
'surplus' is the definition.
(I've seen this before)
'engineers left cars round' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'left' becomes 'L' (common abbreviation).
'cars' becomes 'Audis' (Audi is a car manufacturer).
'round' says the letters should be written in reverse.
'l'+'audis'='laudis'
'laudis' in reverse letter order is 'sidual'.
're'+'sidual'='RESIDUAL'
(Other definitions for residual that I've seen before include "Concerning what is left over" , "surplus" , "unused" , "Left over as a remainder" , "Leftover - used rail (anag)" .)