Sweetener your drink contains (5)
I believe the answer is:
syrup
'sweetener' is the definition.
(syrup is a kind of sweetener)
'your drink contains' is the wordplay.
'your' becomes 'yr'.
'drink' becomes 'sup' (I've seen this before).
'contains' means one lot of letters goes inside another.
'yr' inserted into 'sup' is 'SYRUP'.
(Other definitions for syrup that I've seen before include "linctus" , "Sugary ingredient" , "Treacle" , "Sweet stuff" , "Thick sweet sticky liquid" .)