Model, short of time, gets soft drink (3)
I believe the answer is:
Here is my best explanation:
'drink' is the definition.
(sipping is a kind of drinking)
'model short of time gets soft' is the subsidiary indication.
'model' becomes 'sit' ('sit' can be a synonym of 'model').
'short of' is a deletion indicator.
'time' becomes 't'.
'gets' is a charade indicator (letters next to each other).
'soft' becomes 'p' (**).
'sit' with 't' taken out is 'si'.
(Other definitions for sip that I've seen before include "Drink gingerly", "Little drink", "Drink very small mouthfuls", "Drink slowly", "Thimbleful", "Small mouthful of liquid", "A small taste", "Take a little of a drink", "Very small drink" and "Take in slowly".)