Speed in system, post-mortem reveals (5)
I believe the answer is:
tempo
'speed' is the definition.
(I've seen this before)
'system post-mortem reveals' is the wordplay.
'reveals' says the answer is hidden in the clue.
'TEMPO' is hidden within 'system post mortem'.
'in' is the link.
(Other definitions for tempo that I've seen before include "Musical beat or rhythm" , "Speed at which a musical composition is to be played" , "Speed or beat, musically" , "Time (music)" , "Muscial time" .)