Devastating impediment for prince (6)
I believe the answer is:
lethal
'devastating' is the definition.
'impediment for prince' is the wordplay.
'impediment' becomes 'let' ('let' could historically mean a hindrance).
'for' says to put letters next to each other.
'prince' becomes 'hal' (I've seen this before).
'let'+'hal'='LETHAL'
(Other definitions for lethal that I've seen before include "Capable of causing death" , "Enough to cause death" , "Liable to cause death" , "describing a killer?" , "Able to cause death" .)