Researcher, say, one going into trail of evidence, is right at last (9)
I believe the answer is:
scientist
'researcher say' is the definition.
(researcher is a kind of scientist)
'one going into trail of evidence is right at last' is the wordplay.
'one going into' is an insertion indicator.
'trail' becomes 'scent' (I've seen this before).
'of evidence' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'at last' suggests the final letters.
The last letter of 'right' is 't'.
'scent' enclosing 'i' is 'scient'.
'scient'+'is'+'t'='SCIENTIST'
Can you help me to learn more?
(Other definitions for scientist that I've seen before include "Chemist or physicist, for instance" , "`Tis nicest kind of specialist" , "One studying nature or physics, say" , "Eg, a chemist" , "Researcher - boffin" .)