Put a relative under street arrest (6)
I believe the answer is:
stasis
'arrest' is the definition.
(I've seen this before)
'put a relative under street' is the wordplay.
'put' indicates putting letters inside.
'relative' becomes 'sis' (short for sister).
'under' means one lot of letters go next to another (below, in a down clue).
'street' becomes 'st' (abbreviation).
'sis' after 'st' is 'stsis'.
'a' put into 'stsis' is 'STASIS'.
(Other definitions for stasis that I've seen before include "Condition of arrested progress" , "Stoppage of blood flow in body" , "State of inactivity where nothing moves" , "Period of equilibrium" , "Assist with stagnant condition" .)