Possibly warm end of September (5)
I believe the answer is:
ember
'possibly warm end of september' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'warm end of september' is the wordplay.
'warm' becomes 'embe' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'end of' suggests the final letters.
The last letter of 'september' is 'r'.
'embe'+'r'='EMBER'
Can you help me to learn more?
(Other definitions for ember that I've seen before include "Smouldering piece left over from a fire" , "Glowing cinder" , "Glowing, smouldering piece of coal or wood" , "Glowing remainder of a fire" , "showing residual fire" .)