Return of Pétain and Quisling, say, wanting power increase (4,2)
I believe the answer is:
step up
'increase' is the definition.
(I know that step up is a type of increase)
I don't understand the rest of the clue.
(Other definitions for step up that I've seen before include "Sudden increase (in rate, output, etc)" , "Come forward" , "Approach" , "Boost" , "Accelerate - promotion" .)