Move boulder from side to side (4)

I believe the answer is:

rock

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'move boulder' is the definition.
'rock' can be an answer for 'boulder' (I've seen this in another clue). I am not certain of the 'move' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for rock that I've seen before include "to soothe baby" , "Move side to side" , "Boulder; music" , "Lull (baby); 50s pop" , "Large stone - type of music" .)

I've seen this clue in The Irish Independent.

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