Modern decree keeping first of these unimpaired (6)
I believe the answer is:
intact
'unimpaired' is the definition.
(similar in meaning)
'modern decree keeping first of these' is the wordplay.
'modern' becomes 'in' (I've seen this in other clues).
'decree' becomes 'act' (I've seen this in other clues).
'keeping' is an insertion indicator (some letters kept inside).
'first of these' becomes 't' (1st letter of 'these').
'in'+'act'='inact'
'inact' enclosing 't' is 'INTACT'.
(Other definitions for intact that I've seen before include "Flawless" , "Not damaged in any way" , "Complete - unbroken" , "All in one piece" , "Complete - undamaged" .)