Started a fire again with a quantity of petrol (5)
I believe the answer is:
litre
'a quantity of petrol' is the definition.
(I've seen this in another clue)
'started a fire again' is the wordplay.
'started a fire' becomes 'lit' (I've seen this in another clue).
'again' becomes 're' (prefix meaning 'again').
'lit'+'re'='LITRE'
'with' acts as a link.
(Other definitions for litre that I've seen before include "Amount of liquid" , "Metric measure of capacity" , "Measure of liquid volume" , "French volume" , "non-imperial measure" .)