PC sat up with entrance of knight (7)
I believe the answer is:
desktop
'pc' is the definition.
(I've seen this before)
'sat up with entrance of knight' is the wordplay.
'sat' becomes 'posed' ('pose' can be a synonym of 'sit').
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'with' is an insertion indicator.
'entrance of knight' becomes 'kt' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'posed' back-to-front is 'desop'.
'desop' placed around 'kt' is 'DESKTOP'.
Can you help me to learn more?
(Other definitions for desktop that I've seen before include "Publishers'" , "working area in office" , "Kind of PC (for publishing?)" , "Like a computer system" , "Work surface; computer type" .)