Gold beats yttrium in stiff test (7)
I believe the answer is:
autopsy
'stiff test' is the definition.
(test done on a corpse ie a 'stiff')
'gold beats yttrium' is the wordplay.
'gold' becomes 'au' (Au is the chemical symbol for gold).
'beats' becomes 'tops' (both can mean to outdo or better).
'yttrium' becomes 'y' (chemical symbol).
'au'+'tops'+'y'='AUTOPSY'
'in' is the link.
(Other definitions for autopsy that I've seen before include "Examination to determine cause of death" , "PM" , "A post-mortem examination of corpse" , "final critical analysis" , "final examination" .)