Sudden power to punch a bully no end (6)
I believe the answer is:
abrupt
'sudden' is the definition.
(similar in meaning)
'power to punch a bully no end' is the wordplay.
'power' becomes 'p' (symbol in physics).
'to punch' means one lot of letters goes inside another (some letters punch a hole in others).
'bully no end' becomes 'brut' (I can't explain this - if you can you should believe this answer much more).
'a'+'brut'='abrut'
'p' inserted within 'abrut' is 'ABRUPT'.
Can you help me to learn more?
(Other definitions for abrupt that I've seen before include "Terse and sudden" , "Sudden, maybe discourteous" , "Curt and brief" , "Sudden - blunt" , "Hurried or curt" .)