Yeah, I would clean it up on the inside/outside of both joining pipes. Then add a thin layer of oil. You can leave the clip on so that when it goes in properly, it will 'click' in place. Otherwise, check to see if the clip is somehow getting in the way of the pipe going into place.
Why was the pipe removed? If it is easier, you could remove the other end, put this end in FIRST, then do the other side. May be easier, may be more difficult. Also, might give you more leeway if you have the mounting bolts disconnected so you can move it around more.