Unaltered under street arrest (6)
I believe the answer is:
stasis
'arrest' is the definition.
(I've seen this before)
'unaltered under street' is the wordplay.
'unaltered' becomes 'as is' (ie something left 'as is').
'under' means one lot of letters go next to another.
'street' becomes 'st' (abbreviation).
'asis' after 'st' is 'STASIS'.
(Other definitions for stasis that I've seen before include "Inactivity or stoppage of circulation" , "Assist with stagnant condition" , "Assist (anag.)" , "Abnormal state in which the flow of blood is stopped" , "Secret police force's" .)