Thick, compact (5)
I believe the answer is:
dense
This clue is a double definition.
'thick' is the first definition.
(I know that thick can be written as dense)
'compact' is the second definition.
(synonyms)
(Other definitions for dense that I've seen before include "Not very bright" , "Thick; stupid" , "Thick, impenetrable" , "Obtuse - impenetrable" , "Thick, compact or crowded" .)