Stupid group, heavily built (8)
I believe the answer is:
thickset
'heavily built' is the definition.
(I've seen this before)
'stupid group' is the wordplay.
'stupid' becomes 'thick' (similar in meaning).
'group' becomes 'set' (both can mean a collection).
'thick'+'set'='THICKSET'
(Other definitions for thickset that I've seen before include "Concentrated" , "Solidly built; stocky" , "Strapping" , "Compact" , "Beefy" .)