Small drink (3)
(Other definitions for sip that I've seen before include "Take a little of a drink" , "Drink daintily" , "Drink gingerly" , "Tiny amount of drink" , "A small taste" .)
'small drink' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nip that I've seen before include "Feeling cold" , "Dram of spirits" , "Small drink or small bite" , "Small drink of spirits" , "Shot" .)