Announced they'd remain with sibling during period of inactivity (6)
I believe the answer is:
stasis
'period of inactivity' is the definition.
(I've seen this before)
'announced they'd remain with sibling during' is the wordplay.
I cannot really understand how this works, but
'sibling' could be 'sis' (short for sister) and 'sis' is located in the answer.
The remaining letters 'sta' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for stasis that I've seen before include "Assist with stagnant condition" , "Condition of arrested progress" , "lack of activity" , "Assist (anag.)" , "Abnormal state in which the flow of blood is stopped" .)