No.1 in charts, Love Me Do, was not primarily a post-drug experience (8)
I believe the answer is:
comedown
'a post-drug experience' is the definition.
(the experience of coming down off drugs)
'no 1 in charts love me do was not primarily' is the wordplay.
'no 1 in charts' becomes 'c' (1st letter of 'charts').
'love' becomes 'o' (love means zero in tennis).
'primarily' suggests taking the first letters.
The initial letters of 'was not' is 'wn'.
'c'+'o'+'me'+'do'+'wn'='COMEDOWN'
(Other definitions for comedown that I've seen before include "Descent to a lower status" , "Anticlimax" , "lessening of its effect?" , "Feeling of disappointment" , "Loss of status, disappointment" .)