Immediately starts selling tasteless rubbish (4)
I believe the answer is:
stat
'immediately' is the definition.
(short for Latin statim)
'starts selling tasteless rubbish' is the wordplay.
'starts' indicates taking the first letters.
'tasteless rubbish' becomes 'tat' (I've seen this before).
The first letter of 'selling' is 's'.
's'+'tat'='STAT'
(Other definitions for stat that I've seen before include "accepted numerical fact" , "Doctor's immediately" , "Photocopy" , "figure used in estimates" , "copy" .)