As a drink, only a little one, obviously (3)
I believe the answer is:
tot
'as a drink only a little one obviously' is the definition.
I can't judge whether this definition defines the answer.
'as a drink only a little one' is the wordplay.
'as' becomes 'tt' (I am not sure about this - if you are sure you should believe this answer much more).
'a drink' means one lot of letters goes inside another.
'only a little' suggests taking the first letters.
The initial letter of 'one' is 'o'.
'tt' placed around 'o' is 'TOT'.
Can you help me to learn more?
(Other definitions for tot that I've seen before include "Child's" , "nipper" , "Small amount of spirits" , "Add up like infant" , "Youngster; short drink" .)