Liven (up) (3)
'liven up' is the definition.
(I've seen this in another clue)
This is all the clue.
(Another definition for energize that I've seen is " power".)
'liven up' is the definition.
(as in a pep talk)
This is the entire clue.
(Other definitions for pep that I've seen before include "Energy or enthusiasm" , "Get-up-and-go" , "Palindromic energy or vim" , "Energy, zip" , "Energy, liveliness" .)