Witness woodcutter's up-and-down motion (6)
I believe the answer is:
seesaw
'up-and-down motion' is the definition.
(I've seen this before)
'witness woodcutter's' is the wordplay.
'witness' becomes 'see' (I've seen this before).
'woodcutter' becomes 'saw'.
'see'+'saw'='SEESAW'
(Other definitions for seesaw that I've seen before include "Lurch up and down" , "Teeter-totter" , "Children's up-and-down play structure" , "Playground furniture" , "Move up and down on the plaything" .)