Sweet biscuit or taco oddly dunked in drink (9)
I believe the answer is:
shortcake
'sweet biscuit' is the definition.
(type of biscuit)
'or taco oddly dunked in drink' is the wordplay.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
'dunked in' indicates putting letters inside.
'drink' becomes 'shake' (milkshake).
The alternating letters of 'taco' are 'tc'.
'or'+'tc'='ortc'
'ortc' put into 'shake' is 'SHORTCAKE'.
(Other definitions for shortcake that I've seen before include "Rate shock (anag)" , "Rich biscuit; American dessert" , "Sweet biscuit - rat chokes (anag)" , "Dessert from America" , "Rich type of biscuit" .)