Boundless glee inside following gag (7)
I believe the answer is:
silence
'gag' is the definition.
(I know that gag is a more specific form of silence)
'boundless glee inside following' is the wordplay.
'boundless' indicates the central letters (the limits or bounds of the word are removed).
'inside' means one lot of letters goes inside another.
'following' becomes 'since' (since can mean following or after).
The middle of 'glee' is 'le'.
'le' put into 'since' is 'SILENCE'.
(Other definitions for silence that I've seen before include "Period of peace" , "Gag" , "There's nothing heard" , "Quietude" , "Secrecy; stillness" .)