[{"tid":1,"matches":[{"dord":1,"matches":[{"tord":1,"score":11},{"tord":2,"score":12}]},{"dord":2,"matches":[{"tord":1,"score":21},{"tord":2,"score":22}]}]}]

db.collection.aggregate([{"$set":{"member":{"$let":{"vars":{"m1":{"$arrayElemAt":["$matches",1]}},"in":{"$arrayElemAt":["$$m1.matches",1]}}}}},{"$project":{"member":1}}])