Principal way plea is amended (6)
I believe the answer is:
staple
'principal' is the definition.
(thesaurus)
'way plea is amended' is the wordplay.
'way' becomes 'st'.
'is amended' indicates anagramming the letters.
'plea' anagrammed gives 'aple'.
'st'+'aple'='STAPLE'
(Other definitions for staple that I've seen before include "Principle - fixer" , "Important; paper fastener" , "Fastener - principal" , "Basic sort of plates" , "Fastening wire" .)