Stop but don't sit down yet (10)
I believe the answer is:
'stop' is the definition.
(I know that standstill is a type of stop)
'down yet' is the wordplay.
'down' becomes 'stand' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'yet' becomes 'still' (synonyms).
'stand'+'still'='STANDSTILL'
'but don't sit' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for standstill that I've seen before include "Situation without movement or activity" , "arrest" , "Complete stop" , "complete halt" , "stoppage" .)