I've lost the drift in bottom right corner of this one.
8d Commercial mail for outlying business centre (7,4) -r-d-n- ----
16d .... a learner leaving the Antipodes for this country (7) a------
11a and 17d Initial piece of instruction used in the services (5,6) first -e----
21d A forte for describing how speech is delivered (5) no letters
20a Clear-cut description of doubtful practices (5) no letters
23a Do they enlarge on selections from text-books (7) no letters
24a Aural devices assist in the trials (7-4) -e-r--- ----
Please help