Heavily built group of little intelligence (8)
I believe the answer is:
thickset
This might be a double definition.
'heavily built' is the first definition.
(I've seen this before)
'group of little intelligence' is the second definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?
(Other definitions for thickset that I've seen before include "Beefy" , "Compact" , "Solidly built; stocky" , "Concentrated" , "sturdy" .)