First letter in it I left under top ad (7)
I believe the answer is:
initial
'first letter' is the definition.
(I've seen this before)
'in it i left under top ad' is the wordplay.
'left' becomes 'l' (common abbreviation).
'under' is a charade indicator (letters next to each other).
'top' indicates taking the first letters.
The first letter of 'ad' is 'a'.
'l' after 'a' is 'al'.
'in'+'it'+'i'+'al'='INITIAL'
(Other definitions for initial that I've seen before include "Relating to the beginning" , "Endorse" , "E in Eileen" , "Introductory" , "Occurring at the beginning" .)