Silence adult, senile (4)
I believe the answer is:
gaga
'senile' is the definition.
('gaga' can be a synonym of 'senile')
'silence adult' is the wordplay.
'silence' becomes 'gag' (gagging is a kind of silencing).
'adult' becomes 'a'.
'gag'+'a'='GAGA'
(Other definitions for gaga that I've seen before include "Foolishly doting" , "Queen's radio recording" , "Senile - slightly crazy" , "In senile dotage" , "Slang for crazy - apply a gag" .)