I believe the answer is:
'mouthful' is the definition.
(I have seen 'Small mouthful ' mean 'sip' so perhaps 'mouthful' could also mean 'sip')
This is all the clue.
(Other definitions for sip that I've seen before include "Drink, small mouthful" , "Small mouthful of fluid" , "Drink daintily" , "A small taste" , "Drink gingerly" .)