Hi Little Princess!
Like scoop rightfully says you are going outside of the clubbing season, all clubs with the exception of Pacha in Ibiza town will be closed - Pacha is open on Friday and Saturday nights only. Although occasionally special events do happen at nightclubs outside of the season, so be on the watch out for them if you would like to experience the clubs and bear in mind, these can often be virtually unannouced, so keep an ear to the ground.
However the weather is usually still pretty nice, it is likely that you will need warm clothes for the evenings but during the day it could very well still be very warm - the average max at the time you are going is 23c, while the average low is still a respectable 16c.
Many people like to go in October as Ibiza is largely quiet and the locals and expats tend to reclaim the island, resulting in some fun nights out as the bars start to maybe relax their approach of trying to make as much money as they possibly can.
I hope you have a great holiday.