Bustling activity in sandbox on regular basis (3)
I believe the answer is:
ado
'bustling activity' is the definition.
(I've seen this in another clue)
'sandbox on regular basis' is the wordplay.
'on regular basis' means one should take alternating letters (letters selected at regular intervals).
The alternating letters of 'sandbox' are 'ADO'.
'in' acts as a link.
(Other definitions for ado that I've seen before include "Fuss, difficulty" , "'Much ... About Nothing, Shakespeare play (3)'" , "Flap" , "carry-on" , "Fuss or bustle" .)