Move up and down or back and forth (6)
I believe the answer is:
seesaw
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">move up and down</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">back and forth</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ ></span>
(Other definitions for seesaw that I've seen before include "Plank children play on" , "Alternate" , "that has its ups and downs" , "Rise and fall" , "Playground device" .)