A bully briefly seizing power will be short (6)
I believe the answer is:
abrupt
'short' is the definition.
(abrupt can mean short or terse)
'a bully briefly seizing power' is the wordplay.
'bully briefly' becomes 'brut' (I can't justify this - if you can you should believe this answer much more).
'seizing' is an insertion indicator (inserted letters are seized).
'power' becomes 'p' (symbol in physics).
'brut' placed around 'p' is 'brupt'.
'a'+'brupt'='ABRUPT'
'will be' is the link.
Can you help me to learn more?
(Other definitions for abrupt that I've seen before include "Short and brusque" , "Curt and brief" , "Sudden, brusque" , "Snap!" , "Impolite" .)