Time's up to start sorting issues (5)
I believe the answer is:
emits
'issues' is the definition.
(I've seen this in another clue)
'time's up to start sorting' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'to start' indicates taking the first letters.
The first letter of 'sorting' is 's'.
'time' backwards is 'emit'.
'emit'+'s'='EMITS'
(Other definitions for emits that I've seen before include "Smite (anag)" , "releases" , "Gives off, discharges" .)