Plot against father assuming power (8)
I believe the answer is:
conspire
'plot' is the definition.
(I know that conspire is a type of plot)
'against father assuming power' is the wordplay.
I cannot quite understand how this works, but
'against' could be 'on' (I've seen this in other clues) and 'on' is found in the answer.
'power' could be 'p' (symbol in physics) and 'p' is located in the answer.
'father' could be 'sire' (synonyms) and 'sire' is found in the leftover letters.
A single letter 'c' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for conspire that I've seen before include "Plot secretly" , "Act secretly with others" , "So prince is given to plan secretly with others" , "implicated in plot" , "'Plot, connive (8)'" .)