Gold best security, ultimately, shown by late investigation (7)
I believe the answer is:
autopsy
'late investigation' is the definition.
'autopsy' can be an answer for 'investigation' (autopsy is a kind of investigation). I am not certain of the 'late' bit.
'gold best security ultimately' is the wordplay.
'gold' becomes 'au' (Au is the chemical symbol for gold).
'best' becomes 'tops'.
'ultimately' says to take the final letters.
The final letter of 'security' is 'y'.
'au'+'tops'+'y'='AUTOPSY'
'shown by' acts as a link.
Can you help me to learn more?
(Other definitions for autopsy that I've seen before include "Dissection of a body to determine the cause of death" , "A post-mortem examination of corpse" , "Examining body" , "Pays out (anag.)" , "Post-mortem inspection" .)