Ignited some fires 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)
'ignited some fires' is the wordplay.
'ignited' becomes 'lit' (synonyms).
'some fires' becomes 're' (I can't justify this - if you can you should give a lot more credence to this answer).
'lit'+'re'='LITRE'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for litre that I've seen before include "Mcc?" , "quantity of drink" , "Metric liquid measure" , "One cubic decimetre" , "It's equal to about 1.75 pints" .)