A Parisian engaged in flimsy, crazy thinking (6)
I believe the answer is:
lunacy
'crazy thinking' is the definition.
(being a lunatic)
'a parisian engaged in flimsy' is the wordplay.
'a parisian' becomes 'un' ('a' in French).
'engaged in' means one lot of letters goes inside another.
'flimsy' becomes 'lacy' (lace is a flimsy material).
'un' inserted inside 'lacy' is 'LUNACY'.
(Other definitions for lunacy that I've seen before include "Moon madness" , "Madness, insanity" , "Extreme folly, insanity" , "Tomfoolery" , "State of madness" .)