Exert, as power (5)
I believe the answer is:
wield
'exert as power' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for wield that I've seen before include "Handle - manage" , "Use (weapon, authority)" , "Brandish a weapon or exert power" , "Hold and use (a weapon or tool)" , "exercise" .)