Rigid bar pivoting about a fulcrum (5)
I believe the answer is:
lever
'rigid bar pivoting about a fulcrum' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lever that I've seen before include "Revel in moving bar" , "Crowbar" , "Lifting bar pivoted on a fulcrum" , "Exert pressure" , "Kind of bar for prising or lifting" .)