Rigid support (4)
I believe the answer is:
prop
'rigid support' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for prop that I've seen before include "Moveable object used on stage" , "A rigid support - 37 Across is one" , "Support; object used on stage" , "One is usually among the strongest and heaviest rugby players" , "Hold (up), support" .)