A few quotes supporting my above claim:
-Alexander Stephens, Confederate Vice President, in his "Cornerstone Speech"
-Mississippi's Secession Declaration
-South Carolina's Declaration of Immediate Causes of Secession
I can easily provide more for anyone still feeling doubtful.
Slavery was "a cause". It was far from the primary cause. There were several others.
Regardless, it does not change the matter that it was not, by definition, a civil war.