Stupid not to be runny (5)
I believe the answer is:
thick
'stupid' is the definition.
(similar in meaning)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for thick that I've seen before include "Broad; dense" , "Stupid (as two short planks?)" , "But surely not all thieves are" , "Foolish" , "deficient in understanding" .)