Benders observed moving back when faced with king (5)
I believe the answer is:
knees
'benders' is the definition.
(things which often 'bend')
'observed moving back when faced with king' is the wordplay.
'observed' becomes 'seen' ('seen' can be a synonym of 'observed').
'moving back' says the letters should be written backwards.
'when faced with' says to put letters next to each other (the face is the beginning of the word).
'king' becomes 'k'.
'seen' back-to-front is 'nees'.
'nees' put after 'k' is 'KNEES'.
(Other definitions for knees that I've seen before include "Legs' joints" , "Leg joints" , "Hinge joints" , "Lap" , "Hinged joints" .)