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 "Very small drink", "Smallest drink", "Take a little of a drink", "Drink very small mouthfuls", "A small taste", "Drink slowly", "Small mouthful of liquid", "Thimbleful", "Drink in tiny mouthfuls" and "Small mouthful of fluid".)