Irish town briefly supporting year-end event (9)
I believe the answer is:
decathlon
'event' is the definition.
(decathlon is a kind of event)
'irish town briefly supporting year-end' is the wordplay.
'irish town' becomes 'Athlone' (town in Ireland).
'briefly' means to remove the last letter.
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'year end' becomes 'dec' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'athlone' with its final letter taken off is 'athlon'.
'athlon' after 'dec' is 'DECATHLON'.
Can you help me to learn more?
(Other definitions for decathlon that I've seen before include "Sporting event with 10 disciplines" , "Latched on to athletic competition of 10 events" , "Series of events" , "sporting contest" , "Athletics contest" .)