Go up and down (6)
'go up and down' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for seesaw that I've seen before include "Rise and fall" , "Piece of playground equipment" , "Plaything with up-and-down movement" , "Change back and forth rapidly and repeatedly" , "keep alternating" .)
I believe this is a double definition.
'go' is the first definition.
(both can mean energy or vitality)
'up and down' is the second definition.
(Other definitions for pep that I've seen before include "Ginger; tax-free investment" , "Get-up-and-go" , "savings plan" , "Energy, zip" , "Palindromic energy or vim" .)
'go up and down' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for yo-yo that I've seen before include "Move up and down" , "Toy (with ups-and-downs!)" , "Toy - fluctuate wildly" , "Toy again in fashion" , "Toy consisting of a reel and string" .)