Those heading up promotions should sell things over here! (4)
I believe the answer is:
psst
'over here' is the definition.
(I've seen this in another clue)
'those heading up promotions should sell things' is the wordplay.
'those heading up' suggests taking the first letters.
The initial letters of 'promotions should sell things' is 'PSST'.
(Other definitions for psst that I've seen before include "Don't tell anyone" , "way of attracting attention" , "Listen!" , "Interjection to draw attention" , "Interjection to attract someone's attention without alerting others" .)