Principal design for plates (6)
I believe the answer is:
staple
'principal' is the definition.
(I've seen this in another clue)
'design for plates' is the wordplay.
'design for' indicates an anagram (I've seen 'designed for' mean this (letters re-designed)).
'plates' with letters rearranged gives 'STAPLE'.
(Other definitions for staple that I've seen before include "wool" , "Paper fastener found on plates" , "Fastening device" , "Raw material" , "Metal fixer" .)