Change design and sit once more (7)
I believe the answer is:
remodel
This clue is a double definition.
'change design' is the first definition.
(to model or design something again)
'sit once more' is the second definition.
('model' can be a synonym for 'sit' so perhaps 'remodel' is OK for 'sit once more')
'and' acts as a link.
(Other definitions for remodel that I've seen before include "style again" , "Change the shape" , "Change the form of" , "Reshape" , "Make over" .)