The king's plastic foot? (5)
I believe the answer is:
ruler
'the king's' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ruler that I've seen before include "Source of line" , "measure applied in school?" , "Person in charge of country" , "Sovereign, for example" , "Person commanding" .)