what could have been a very difficult case for the defense.在

(Douglas said the guilty plea brings to a close主句)( what could have been a very difficult case for the defense宾语从句）.
1、分析：这是一个主从复合句,这里 what 引导的从句作主句谓语 brings...to a close 的宾语,即这个从句是宾语从句,what 本身在从句中作主语；
2、试翻译：Douglas 说,有罪辩护使得本来对辩方律师非常难打的一个案件宣告结束.

名词性从句 中的 主语从句

宾语从句，作为close的宾补，解释close的内容，what在从句中作主语