Couple of grams American snorted ... crack? (3)
I believe the answer is:
gag
'crack?' is the definition.
(both can mean a joke)
'couple of grams american snorted' is the wordplay.
'couple of' shows that letters should be duplicated.
'grams' becomes 'g' (abbreviation).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'snorted' is an insertion indicator (inserted letters are snorted up).
'g' duplicated is 'gg'.
'gg' enclosing 'a' is 'GAG'.
(Other definitions for gag that I've seen before include "Muffle with slang joke" , "Make quiet" , "Funny" , "Silence the joke" , "Retch at the joke?" .)