It's actually an adverb.
suc·cinct·ly
sə(k)ˈsiNG(k)tlē,sə(k)ˈsiNGk(t)lē/
adverb
in a brief and clearly expressed manner.
"one word succinctly describes the economy's performance: unbalanced"
Douchebag.....
You're right, adverb it is.
Classy, too.