Movable seat required for the biker

I believe the answer is:

rocker

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'movable seat' is the definition.
'rocker' can be an answer for 'seat' (rocker is a kind of seat). I'm unsure of the 'movable' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for rocker that I've seen before include "Nineteen sixties mod counterpart" , "cyclist in leathers" , "Mad, off it; 50s biker" , "Mod's 1960s rival - curved support under chair legs" , "Corker (anag)" .)

I've seen this clue in The Irish Independent.

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