Trail is enthralling one tense researcher (9)
I believe the answer is:
scientist
'researcher' is the definition.
(I've seen this before)
'trail is enthralling one tense' is the wordplay.
'trail' becomes 'scent' (an animal's trail is its scent).
'enthralling' means one lot of letters goes inside another.
'one' becomes 'I' (Roman numeral).
'tense' becomes 't' (abbreviation used in many dictionaries).
'scent'+'is'='scentis'
'scentis' going around 'i' is 'scientis'.
'scientis'+'t'='SCIENTIST'
(Other definitions for scientist that I've seen before include "Chemist or physicist, for instance" , "Researcher - boffin" , "Inset tics for this learned one" , "someone in laboratory?" , "It's nicest for one engaged in specialist studies" .)