activity.kt

class MainActivity : Activity, Adapter.OnCellClickListener {
    
    override fun onCreate() {
        recyclerView.setAdapter(Adapter(this)) // this refers to OnCellClickListener
    }
    
    override fun onCellClick(item: ChattrItem) {
        // hoooray i have access to ChattrItem!!!
    }
}

adapter.kt

class Adapter(private val listener: OnCellClickListener) {

    override fun onCreateViewHolder() {...}
    
    override fun onBindViewHolder() {...}
    
    override fun getItemCount() {...}
    
    interface OnCellClickListener {
        fun onCellClick(item: ChattrItem)
    }
    
    inner class CustomViewHolder(view: View) {
        fun bind(item: ChattrItem) {
            view.setOnClickListener {
                listener.onCellClick(item) // pass ChattrItem to this interface
            }
        }
    }
}