Speed discovered during post-mortem, possibly (5)
I believe the answer is:
tempo
'speed' is the definition.
(thesaurus)
'during post-mortem possibly' is the wordplay.
'during' indicates a hidden word.
'TEMPO' is hidden within 'post mortem possibly'.
'discovered' acts as a link.
(Other definitions for tempo that I've seen before include "Pace, rate" , "Pace - pacing" , "Speed of a musical piece" , "Speed at which piece of music is played" , "musical quality?" .)