How to reclaim unused space from clustered index table
You may need to run an index Rebuild (not reorganise) on the clustered index - note in standard edition this will lock the table for writes for the duration of the rebuild (and reads may be affected), which could be some hours. 

Otherwise a simple DBCC SHRINKFILE might work, but be aware of this: https://littlekendra.com/2016/11/08/shrinking-sql-server-data-files-best-practices-and-why-it-sucks/