Vacillate endlessly and move from side to side (4)
I believe the answer is:
wave
'move from side to side' is the definition.
The definition and answer can be both to do with motion as well as being verbs in their base form.
Maybe you can see a link between them that I don't see?
'vacillate endlessly' is the wordplay.
'vacillate' becomes 'waver' (synonyms).
'endlessly' means to remove the last letter.
'waver' with its final letter removed is 'WAVE'.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for wave that I've seen before include "Move hand in farewell" , "Hand signal; surge" , "Move hand to and fro as greeting" , "Roller, move to and fro" , "may have a crest at the top?" .)