Alight, burning (5)
I believe the answer is:
afire
I believe this is a double definition.
'alight' is the first definition.
(synonyms)
'burning' is the second definition.
(on fire)
(Other definitions for afire that I've seen before include "in flames" , "Alight" , "Burning" , "Ablaze" , "Blazing - excited" .)