Festivity starts in garden and lasts ages (4)
I believe the answer is:
gala
'festivity' is the definition.
(I know that gala is a type of festivity)
'starts in garden and lasts ages' is the wordplay.
'starts in' indicates taking the first letters.
The first letters of 'garden and lasts ages' is 'GALA'.
(Other definitions for gala that I've seen before include "Festive event" , "Mix" , "Festivity - celebration" , "sporting event" , "A festive occasion, perhaps of sporting nature" .)