Investigator going up first round Olympic site (6)
I believe the answer is:
poirot
'investigator' is the definition.
(fictional detective Hercule Poirot)
'going up first round olympic site' is the wordplay.
'going up' says the letters should be written in reverse.
'first' becomes 'top' (synonyms).
'round' means one lot of letters goes inside another.
'olympic site' becomes 'rio' (site of the o0t6 Summer Olympics).
'top' going around 'rio' is 'toriop'.
'toriop' backwards is 'POIROT'.
(Other definitions for poirot that I've seen before include "... and her male one" , "10 [SLEUTH]" , "Hercule . . . . . ., Agatha Christie's detective" , "Fill; live in" , "Hercule --, fictional detective" .)