Youngster put up with crap Fiat (6)
I believe the answer is:
diktat
'fiat' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for diktat that I've seen before include "Official order" , "Forceful order that must be obeyed" , "Harsh order or decree" , "Categorical decree" , "imposed order" .)