Stoppage in force once beginning to slow (6)
'stoppage' is the definition.
(I've seen this before)
'force once beginning to slow' is the wordplay.
'force once' becomes 'stasi' (secret police of East Germany).
'beginning to' suggests taking the first letters.
The first letter of 'slow' is 's'.
'in' is the link.
(Other definitions for stasis that I've seen before include "Standstill", "Stoppage of circulation or stagnation", "Enduring state", "Inactive state", "Assist with stagnant condition".)