Drink up a bit snappishly (3)
I believe the answer is:
sip
'drink' is the definition.
(sipping is a kind of drinking)
'up a bit snappishly' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'a bit' indicates the answer is hidden within the clue (I've seen 'bit of' mean this).
'pis' can be found hidden inside 'snappishly'.
'pis' reversed gives 'SIP'.
(Other definitions for sip that I've seen before include "A small taste" , "Drink, small mouthful" , "Small mouthful of liquid" , "Drink daintily" , "Drink slowly" .)