Solution to 13A has been deliberately left unsolved and is to be answered only by a non-regular / novice commenter, with proper annotation. Those who have answered earlier in the week, please give others a chance.
ACROSS
1 Wise men join spies with new illusionist (8) MAGICIAN {MAGI}{CIA}{N}
5 Like persuasive force, retreating to Thor’s home (6) ASGARD {AS}{DRAG<=}
9 Diplomacy protects royal leader returning with impetus (8) TRACTION {T{R}ACT}{NO1<=}
10 Auditor’s breakfast item in series (6) SERIAL (~cereal)
12 Bowled out old Australian captain in command (5) ORDER bORDER
13 Lawless, start unit that could protect small companies (9) A?T?T?U?T (Adendun - ANTITRUST* - See comments)
14 Arrive missing a day of fun (6) COMEDY {COME}{DaY}
16 Society carried by superhero like Tendulkar, say (7) BATSMAN {BAT{S}MAN}
19 It could be used to portray saint without top drawing aid (7) STENCIL {ST}{pENCIL}
21 Stage about right for remark (6) PHRASE {PH{R}ASE}
23 Period returning interrogators had to eliminate (9) ERADICATE {ERA}{CID<=}{ATE}
25 Society with a cause for worry in shock (5) SCARE {S}{CARE}
26 Morph a news segment featuring child without parents (6) ORPHAN [T]
27 Container with shredded old mail that contains two terms (8) BINOMIAL {BIN}{O+MAIL}*
28 Laidback about measure on prohibition (6) DENIAL {D{EN}IAL<=}
29 Curtailed choice to stay inside drunk by evening (8) TWILIGHT {T{WILl}IGHT}
DOWN
1 Saw emergency room cover ordinary streak of light (6) METEOR {MET}{E{O}R}
2 Impressive broadcast organised (9) GRANDIOSE*
3 Attend to former American president right away (5) CATER CArTER
4 A routine king wasted with unknown divergence (7) ANOMALY {A}{NOrMAL}{Y}
6 Doctor protects a witness (9) SPECTATOR*
7 A roller primarily unused is so long (5) ADIEU {A}{DIE}{Un...d}
8 Making thinner bore frames until damaged (8) DILUTING {DI{UNTIL*}G}
11 Put out smoke at first with water container (4) STUB {Sm..e}{TUB}
15 Each land I wandered for dish (9) ENCHILADA*
18 It soared around minor planet (8) ASTEROID*
20 Principal element (4) LEAD [DD]
21 Producer’s primary critique for trailer (7) PREVIEW {Pr...r}{REVIEW}
22 Magic initially lacking energy takes first shot (6) PELLET {sPELL}{E}{Ta..s}
24 Was penthouse concealing a tree? (5) ASPEN [T]
Reference List
New = N, Royal = R, Bowled = B, Society = S, Right = R, Old = O, Measure = EN, Ordinary = O, King = R, Unknown = Y, Energy = E, Small = S, Large = L