Set fire to about the measured amount (5)
I believe the answer is:
litre
'measured amount' is the definition.
'litre' can be an answer for 'amount' (litre is a kind of amount). I'm not sure about the 'measured' bit.
'set fire to about' is the wordplay.
'set fire' becomes 'lit' (as in lighting a fire).
'to about' becomes 're' (regarding).
'lit'+'re'='LITRE'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for litre that I've seen before include "Fluid measure" , "French volume" , "A pint or two" , "Metric liquid measure" , "quantity of drink" .)