No 12638, Wednesday 29 May 2019, Arden

ACROSS
1   Need song composed in chaste French? (7,2,5) CHANSON DE GESTE {NEED+SONG}* in {CHASTE} &lit
10 Reach as omentum gets a momentum initially (5) GAMUT {G{A}{Mo...m}UT}
11 First test venue — sun set, also spread over morning (3,6) LOS ALAMOS {SOL<=}{AL{AM}OS*}
12 Spooner ran, chased by bully — that's standard in Russia (3,4) RED FLAG (fled rag to red flag)
13 Lover boy goes from head to toe, completely (7) OVERALL {(-l)OVER{AL}(+l)L}
14 Chamber's keen, starts probing ties (5) CRYPT {CRY}{Pr...g}{Ties}
16 I'd gatecrashed earlier into a place of refreshment (3,6) TEA GARDEN {GATE}*{ARDEN}
19 Team's demand for an additional course (4,5) SIDE ORDER {SIDE}{ORDER}
20 Brilliance amid all the clatter (5) ECLAT [T]
22 Perfect score for outstanding girl — a dish! (7) ANTENNA {AN{TEN}NA}
25 Gas had a new issue (7) NEONATE {NEON}{ATE}
27 Now wearing clothes, everyone's well informed (4-2-3) KNOW-IT-ALL {K{NOW}IT}{ALL}
28 Trace article in Times cover (5) T?I?T {Addendum - TAINT - {T{A}IN}T} - See comments)
29 Sign of impending union battle — will call later (10,4) ENGAGEMENT RING {ENGAGEMENT}{RING}

DOWN
2   A nymph had included a crazy line... (9) HAMADRYAD {H{A}{MAD}{RY}AD}
3   ...on birth of two boys (5) NATAL {NAT}{AL}
4   Duty-bound to detect a scam of sorts, within old book cover (9) OBLIGATED {O}{B}{LI{GATE}D}
5   It's flat opening music (5) DISCO {DISC}{O}
6   Engage organisation, get used to peace and prosperity (6,3) GOLDEN AGE {G{OLD}ENAGE*}
7   A doctor in South American island (5) SAMOA {S{A}{MO}A}
8   Foreign character — any displaced Polynesian? (7) EPSILON POLyNESIan*
9   Spies hid paper on growing fungus (6) AGARIC {CI{RAG}A}<=
15 First results in, counting afresh — winning? (9) TROUNCING {T{Re...s}OUNCING*}
17 Learn about breaking a sound barrier — it will provide excitement (9) ADRENALIN {A}{D{LEARN*}IN}
18 Regularly deal with wild animal — takes time, but it's spotted (9) DALMATIAN {DeAl}{LMA{T}IAN*}
19 Restrain half sleepy reporter plugging in (7) SHACKLE {S{HACK}LEepy}
21 Have to go around, it's agreed (6) TREATY {TR{EAT}Y}
23 Strip it, had a change of heart? (5) THONG TH(-i+o)ONG
24 Embrace the French with fear (5) ALARM {A{LA}RM}
26 Essentially draw the fur (5) OTTER lOTTERy

Reference List
Line = RY(Railway), Old = O, Book = B, Doctor = MO, Paper = RAG,

GRID