Keep around third of fires alight (7)
I believe the answer is:
detrain
'alight' is the definition.
(thesaurus)
'keep around third of fires' is the wordplay.
'keep' becomes 'detain' (I've seen this before).
'around' means one lot of letters goes inside another.
'third of fires' becomes 'r' (I can't justify this - if you can you should believe this answer much more).
'detain' placed around 'r' is 'DETRAIN'.
Can you help me to learn more?
(Other definitions for detrain that I've seen before include "leave car" , "leave coach behind" , "Get off at a station" , "leave Orient Express, perhaps" , "get out of carriage" .)