Present verb makes its irregular conjugation fluctuate (6)
I believe the answer is:
seesaw
'fluctuate' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for seesaw that I've seen before include "Pivoted amusement" , "Up-and-down children's game with plank on barrel" , "vacillate in policy" , "'Move up and down alternately'" , "Children's up-and-down play structure" .)