[{"originalValue":"DELETERANDOMVALUE_Keep_This_Words_DELETERANDOMVALUE"}]

db.collection.aggregate([{$project:{_id:0,splitValue:{$split:["$originalValue","_"]}}},{$project:{cleanedValue:{$slice:["$splitValue",1,{$subtract:[{$size:"$splitValue"},2]}]}}},{$project:{finalValue:{$reduce:{input:"$cleanedValue",initialValue:"",in:{$concat:["$$value",{"$cond":[{$eq:["$$value",""]},"","_"]},"$$this"]}}}}}])