Knight involved in support, upset with tyrant’s initial decree (6)
I believe the answer is:
diktat
'decree' is the definition.
(I've seen this before)
'knight involved in support upset with tyrant's initial' is the wordplay.
'knight' becomes 'k'.
'involved' indicates putting letters inside.
'in support upset' becomes 'dita' (I can't explain this - if you can you should give a lot more credence to this answer).
'with' is a charade indicator (letters next to each other).
'initial' suggests taking the first letters.
The initial letter of 'tyrants' is 't'.
'k' inserted within 'dita' is 'dikta'.
'dikta'+'t'='DIKTAT'
Can you help me to learn more?
(Other definitions for diktat that I've seen before include "Dogmatic statement" , "Decree imposed from above" , "Declaration allowing no opposition" , "Forceful order that must be obeyed" , "A firm statement or decree" .)