Deposing leader in panic is blunder (5)
I believe the answer is:
error
'is blunder' is the definition.
(I've seen this before)
'deposing leader in panic' is the wordplay.
'deposing leader in' means to remove the first letter.
'panic' becomes 'terror' (synonyms).
'terror' with its initial letter taken away is 'ERROR'.
(Other definitions for error that I've seen before include "Trial and ___" , "Slip, mistake" , "Gaffe" , "Bug" , "Miscalculation" .)