it's because your oscillator isn't getting it's phase reset at the note on. Like they said above, raising the attack will hide this, but it's good to know why too. If it's a sample of a sine wave you are using, put the start at the zero-crossing point. If its some kind of synth you are using, look for an oscillator phase sync, or reset phase on trigger feature. You want the sine wave to start at zero (the center) like this: http://www.personal.psu.edu/sul168/images/Sine wave.png
If it does not start at zero, then you have a sudden rise (or fall), which produces the click. Its adding overtones at the very start. You'll notice that a LPF with the cutoff set just above the sine waves frequency will help a lot on removing the click too, but starting the wave at zero is the correct way, if possible.
well the "play with the attack and the release" idea doesnt work ,cause when i increase the attack i hear the click like i've increased nothing ,so if u can tell me (i will post this in the ableton sticky too,but if u know ) where is this phase sync or phase restart in the Analog instrument or the Operator's.I will be thankful !
Do you have more then one oscillator running through different envelopes or something ? It's been a while since I used operator, but raising the attack should always smooth out clicks.
Maybe try raising it a bit more ?
In your synth is there an envelope for the oscillator and also a global amplitude envelope. If so have you raised on the attack and release on both of them. Just checking. Same thing for me in Albino and that's the easy fix.
I'm not too familiar with Ableton's instruments (I'm a Logic man myself). However, you could try bouncing down your file and then placing a fade in and fade out and beginning and end (respectively) of the audio region. This may smooth things out. Ideally, sonic noise (i.e. clicking) is best dealt with at the core of the source (synth), so try messing with the global envelope a little more and if all else fails, try the fade routine previously mentioned. Hope this helps.