Trusty French nobleman, collared by a couple, dumped in Cornish river (8)
I believe the answer is:
fiducial
'trusty' is the definition.
('fiducial' can be similar in meaning to 'trusty')
'french nobleman collared by a couple dumped in cornish river' is the wordplay.
'french nobleman' becomes 'duc' ('duke' in French).
'collared by' means one lot of letters goes inside another (I've seen this in other clues).
'a couple' becomes 'ii' (I've seen this in other clues).
'dumped in' indicates putting letters inside.
'cornish river' becomes 'fal' (River Fal).
'duc' inserted inside 'ii' is 'iduci'.
'iduci' put into 'fal' is 'FIDUCIAL'.
(Another definition for fiducial that I've seen is " Based on trust".)