Subsidiary line subsequently brought in fuel (10)
I believe the answer is:
collateral
'subsidiary' is the definition.
(I've seen this before)
'line subsequently brought in fuel' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'subsequently' becomes 'later' (subsequently means later on).
'brought in' indicates putting letters inside.
'fuel' becomes 'coal' (coal is a kind of fuel).
'l'+'later'='llater'
'llater' inserted within 'coal' is 'COLLATERAL'.
(Other definitions for collateral that I've seen before include "Money put up" , "Confirmatory - recall a lot (anag)" , "Guarantee" , "running side by side" , "Secondary; parallel" .)