Ended a design to take the edge off (6)
I believe the answer is:
deaden
'take the edge off' is the definition.
(thesaurus)
'ended a design' is the wordplay.
'design' indicates an anagram.
'ended'+'a'='endeda'
'endeda' is an anagram of 'DEADEN'.
'to' is the link.
(Other definitions for deaden that I've seen before include "Muffle (sound)" , "Muffle, make muted" , "Make weaker or quieter" , "Damp down; make less sensitive" , "Make numb, deprive of feeling" .)