Thrash with entirely old power (6)
I believe the answer is:
wallop
'thrash' is the definition.
(I've seen this in another clue)
'with entirely old power' is the wordplay.
'with' becomes 'w' (short for with).
'entirely' becomes 'all' (all can mean entirely or completely).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'power' becomes 'p' (symbol in physics).
'w'+'all'+'o'+'p'='WALLOP'
(Other definitions for wallop that I've seen before include "Punch" , "big hit" , "Severe blow" , "ale?" , "Thrash - beer (slang)" .)