Reckless cad stupidly blocks plan (6)
I believe the answer is:
madcap
'reckless' is the definition.
(I've seen this before)
'cad stupidly blocks plan' is the wordplay.
'stupidly' is an anagram indicator (a stupid ordering of the letters).
'blocks' is an insertion indicator (upper walls off glass inside formal).
'plan' becomes 'map' (plan can mean a map of something).
'cad' with letters rearranged gives 'dca'.
'dca' inserted into 'map' is 'MADCAP'.
(Other definitions for madcap that I've seen before include "Insane headgear for wildly impulsive person" , "Dare-devil" , "Foolish, reckless" , "Rash person" , "Foolhardy" .)