Reclaim space from dropped column when there isn't enough space for index rebuild
DBCC CLEANTABLE can only be used to reclaim space left from dropping a variable length column.  Thus it cannot be considered a "good" replacement for rebuilding the table/index.

Since you state that the column you dropped was a fixed length column, I can only surmise there must be some other reason for unused, but allocated space being present in the table; perhaps ghost records or perhaps you actually dropped some other columns as well that actually were variable length.