Eight days of celebration in month on avenue (6)
I believe the answer is:
octave
'eight' is the definition.
(I've seen this before)
'month on avenue' is the wordplay.
'month' becomes 'Oct' (abbreviation for October).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'avenue' becomes 'ave'.
'oct'+'ave'='OCTAVE'
'days of celebration in' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for octave that I've seen before include "Interval from one C to the next C" , "A group of eight consecutive musical notes" , "Series of eight notes in music" , "Twelve semitones" , "Musical interval doubling note frequency" .)