Activity producing nothing after a day (3)
I believe the answer is:
ado
'activity' is the definition.
(ado can mean commotion or activity)
'nothing after a day' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'after' is a charade indicator (letters next to each other).
'day' becomes 'd' (abbreviation for day).
'a'+'d'='ad'
'o' after 'ad' is 'ADO'.
'producing' is the link.
(Other definitions for ado that I've seen before include "Pother" , "Fuss or bustle" , "stew is provided" , "puff ball!" , "Shakespeare: 'Much ... About Nothing'(3) ()" .)