That’s the case model has to lift (3)
I believe the answer is:
tis
'that's the case' is the definition.
(contraction of 'it is')
'model has to lift' is the wordplay.
'model' becomes 'sit' (e.g. for a painting).
'has to lift' is a reversal indicator (in a down clue, letters go up).
'sit' backwards is 'TIS'.
(Other definitions for tis that I've seen before include "Pacific trees" , "I confirm the identity of" , "McCourt work" , "It is shortened" , "Sequel to "Angela's Ashes"" .)