Smartest independent investigator, inspired by trial (7)
I believe the answer is:
tidiest
'smartest' is the definition.
('tidy' can be similar in meaning to 'smart')
'independent investigator inspired by trial' is the wordplay.
'independent' becomes 'i'.
'investigator' becomes 'DI' (abbreviation for detective inspector).
'inspired by' means one lot of letters goes inside another.
'trial' becomes 'test' (synonyms).
'i'+'di'='idi'
'idi' inserted into 'test' is 'TIDIEST'.
(Other definitions for tidiest that I've seen before include "Neatest" , "Extremely organised" , "most trim" , "One orderly more than others" , "smarter than anything" .)