Advertisers initially lean on top celebrities (5)
I believe the answer is:
alist
'top celebrities' is the definition.
(I've seen this before)
'advertisers initially lean on' is the wordplay.
'initially' suggests taking the first letters.
'lean' becomes 'list' (both can mean to tilt).
'on' says to put letters next to each other.
The first letter of 'advertisers' is 'a'.
'a'+'list'='ALIST'
(Other definitions for alist that I've seen before include "Names of sought-after people are on it" , "Most famous celebrities" , "Top celebrities" .)