Silver vibrator decided upon jointly (6)
I believe the answer is:
agreed
'decided upon jointly' is the definition.
I can't judge whether this definition defines the answer.
'silver vibrator' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'vibrator' becomes 'reed' (I've seen this before).
'ag'+'reed'='AGREED'
Can you help me to learn more?
(Other definitions for agreed that I've seen before include "Of one mind" , "Settled by common consent" , "Consented and concurred" , "Did concur" , "Acquiesed, concurred" .)