Suspicion - key is first to corrode (8)
I believe the answer is:
Here is my best explanation:
'suspicion' is the definition.
('distrust' can be a synonym of 'suspicion')
'key is first to corrode' is the subsidiary indication.
'key' becomes 'd' (I've seen this in other clues).
'is first' becomes 'ist'.
'to corrode' becomes 'rust' ('rust' can be a synonym of 'corrode').
(Other definitions for distrust that I've seen before include "Durst sit with lack of confidence?", "Lack confidence in honesty or ability", "show a lack of belief", "Stir dust up and be suspicious", "Be suspicious of", "Have no faith or confidence in", "Be sceptical about", "Regard (person) with suspicion", "Doubt someone's honesty" and "Stir dust (anag.)".)