What are there in the wardrobe = just plain WRONG What's in the wardrobe? = correct expression.

'What' in this sense is singular. So the question has to be 'What's in the wardrobe?' Or you use a plural noun.' What clothes are in the wardrobe?' ' What things are in the wardrobe?' You can answer 'there is' or 'there are' to any of these questions depending on whether your answer is singular or plural.

I think you should treat "what" as a singular. To use "are", you'd need some indication of a plural: "What things are in the wardrobe"? "What clothes are in the wardrobe?"