Mum about 100, sort of senile! (7)
I believe the answer is:
silence
'mum' is the definition.
(as in 'keep mum')
'about 100 sort of senile' is the wordplay.
'about' means one lot of letters goes inside another.
'100' becomes 'c' (Roman numeral).
'sort of' is an anagram indicator.
'senile' with letters rearranged gives 'silene'.
'c' placed into 'silene' is 'SILENCE'.
(Other definitions for silence that I've seen before include "Secrecy; stillness" , "Shut up!" , "There's nothing heard" , "Absence of sound" , "Noiselessness" .)