Busy outside at start (8)
I believe the answer is:
activate
'start' is the definition.
(I've seen this before)
'busy outside at' is the wordplay.
'busy' becomes 'active' (similar in meaning).
'outside' indicates putting letters inside.
'active' placed around 'at' is 'ACTIVATE'.
(Other definitions for activate that I've seen before include "Launch" , "Set off, trip" , "Make operative" , "Start working" , "Give life to" .)