Knock out of a fixed position (8)
I believe the answer is:
dislodge
'knock out of a fixed position' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dislodge that I've seen before include "Prise out of position" , "Remove from established position" , "Gods lied (anag.)" , "Shift" , "put out" .)