Suspicion - key is first to corrode (8)
I believe the answer is:
distrust
'suspicion' is the definition.
('distrust' can be a synonym of 'suspicion')
'key is first to corrode' is the wordplay.
'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'**).
'd'+'ist'+'rust'='DISTRUST'
(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" , "Have no faith or confidence in" .)