Police officers once last in exams — still time! (6)
I believe the answer is:
stasis
'still time' is the definition.
'police officers once last in exams' is the wordplay.
'police officers once' becomes 'stasi' (former East German police).
'last in' indicates one should take the final letters.
The last letter of 'exams' is 's'.
'stasi'+'s'='STASIS'
(Other definitions for stasis that I've seen before include "Secret police force's" , "Assist in state of stagnation" , "Total lack of movement" , "Stoppage of blood circulation" , "Assist with stagnant condition" .)