To put it briefly, not for the stupid (8)
I believe the answer is:
condense
'to put it briefly not for the stupid' is the definition.
I can't tell whether this definition defines the answer.
'not for the stupid' is the wordplay.
'not' becomes 'con' (I've seen this in other clues).
'for' is a charade indicator (letters next to each other).
'the stupid' becomes 'dense' (I've seen this in another clue).
'con'+'dense'='CONDENSE'
Can you help me to learn more?
(Other definitions for condense that I've seen before include "Make more concentrated, or express in fewer words" , "Change from vapour to liquid" , "Make more concise" , "Make more concentrated or compact" , "Decrease, reduce" .)