Please stop saying, “I have a doubt”. This phrase doesn’t exist in English!
When you need more information (clarification) or don’t understand, the normal phrasing is, “I have a question.”
There is an expression “I have my doubts” which means “I am suspicious” or “I don’t trust this situation”, but that is another situation.
Ex: I have a question: does this job offer professional development? (clarification)
Ex: They said the job offers professional development, but I have my doubts. (suspicion)