Litre used to set on fire again (5)
I believe the answer is:
relit
'set on fire again' is the definition.
(I've seen this in another clue)
'litre used' is the wordplay.
'used' indicates an anagram.
'litre' with letters rearranged gives 'RELIT'.
'to' is the link.
(Other definitions for relit that I've seen before include "got going again" , "Again fired" , "in flames again" , "Illuminated again" , "Tiler (anag.)" .)