Thick people of rank dismissing books (7)
I believe the answer is:
viscous
'thick' is the definition.
(similar in meaning)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for viscous that I've seen before include "Thick and gluey, like heavy oil" , "Treacly" , "Resistant to flow" , "Thick, sticky (liquid)" , "Gelatinous" .)