Any number attending earlier Greek festival (4)
I believe the answer is:
agon
'greek festival' is the definition.
The definition suggests a singular noun which matches the answer.
'any number attending earlier' is the wordplay.
'any number' becomes 'n' (mathematical symbol for an unspecified number).
'attending' is a charade indicator (letters next to each other) (to attend can mean to follow).
'earlier' becomes 'ago' (I've seen this before).
'n' put after 'ago' is 'AGON'.
Can you help me to learn more?
(Other definitions for agon that I've seen before include "Struggle" , "Stravinsky ballet" , "Greek contest" , "archaic festival" , "Conflict" .)