Peace slogan involving diligence, no less (7,2,6)
I believe the answer is:
silence is golden
'peace slogan' is the definition.
(I've seen this before)
'involving diligence no less' is the wordplay.
'involving' indicates anagramming the letters.
'diligence'+'no'+'less'='diligencenoless'
'diligencenoless' is an anagram of 'SILENCE IS GOLDEN'.
(Other definitions for silence is golden that I've seen before include "peace slogan" , "Mum's" , "precious saw" .)