Awfully senile, being about 100 and quiet (7)
I believe the answer is:
silence
'quiet' is the definition.
('silence' can be a synonym of 'quiet')
'awfully senile being about 100' is the wordplay.
'awfully' is an anagram indicator.
'being about' indicates putting letters inside.
'100' becomes 'c' (roman numeral).
'senile' is an anagram of 'silene'.
'silene' placed around 'c' is 'SILENCE'.
'and' is the link.
(Other definitions for silence that I've seen before include "Noiselessness" , "Secrecy; stillness" , "It is said to be golden" , "There's nothing heard" , "Nothing said" .)