Have suspicions; first doubt is the open oxidation (8)
I believe the answer is:
distrust
'have suspicions' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'first doubt is the open oxidation' is the wordplay.
'first doubt' becomes 'd' (1st letter of 'doubt').
'open' says to take the initial letters (I've seen 'opens' mean this).
'oxidation' becomes 'rust' (rust is a kind of oxidation).
The first letter of 'the' is 't'.
'd'+'is'+'t'+'rust'='DISTRUST'
Can you help me to learn more?
(Other definitions for distrust that I've seen before include "Doubt someone's honesty" , "Be sceptical about" , "Lack confidence in honesty or ability" , "Durst sit with lack of confidence?" , "Stir dust up and be suspicious" .)