No 12118, Thursday 21 Sep 2017, Gridman

ACROSS
1   Spook not left in marsh (8) FRIGHTEN {F{RIGHT}EN}
5   Dissertation of the relative is cut by half (6) THESIS {THE}{SISter}
10 One takes this to show offence (7) UMBRAGE [CD]
11 At first tipping lumber off cart (7) TUMBREL {Ti...g}{LUMBER*}

12 Damned — two cards come together (6) DEUCED [DD] (Addendum - {DEUCE}{D} - See comments)
13 Not being attractive, Chennai leader leaves and is quite safe (8) HARMLESS cHARMLESS
15 List of tasks of tribal, not ace, getting old (2-2) TO-DO {TODa}{O}
16 Full gathering, at 10, has a ball (10) ATTENDANCE {AT}{TEN}{DANCE}
18 No US goalie would buckle and be smarmy (10) OLEAGINOUS*
20 Indian garment — it's trim without borders (4) SARI {SA}{tRIm}
23 Fitting — a lawyer's put in large hotel room (8) SUITABLE {SUIT{A}{BL}E}
24 What may be thrown off one's head as out-of-date (3-3) OLD-HAT [DD]
26 Gap in job available (7) OPENING [DD]
27 Caution about a kind of sale (7) AUCTION*
28 Area covered once by the big top (6) EXTENT {EX}{TENT}
29 Tie great made-up feathery ornament (8) AIGRETTE*

DOWN
1   Basically it's laid to start building up (10,5) FOUNDATION STONE [CD]
2   Such trains arrive at the railway station (7) INBOUND [CD]
3   Vehement, the man pushed date away (6) HEATED {HE}{DATE*}
4   Flat proceeding stopped short (4) EVEN EVENt
6   It's plain he's adopted mantra to get dame disguised (8) HOMEMADE {H{OM}E}{DAME*}
7   Gush over one who operates (7) SURGEON {SURGE}{ON}
8   With this, all the commercial pitch fails (5,10) SALES RESISTANCE [CD]
9   Indignant son joined forces (7,2) STEAMED UP {S}{TEAMED UP}
14 Studying race hotly gone wayward (9) ETHNOLOGY*
17 Rural city needs rain to fall (8) AGRARIAN {AGRA}{RAIN*}
19 Clear — vie maniacally to make a bit of progress (7) EVIDENT {VIE*}{DENT}
21 A primarily terrible crime by non-believer (7) ATHEIST {A}{Te...e}{HEIST}
22 Deposit of soft, soft material at bottom of river (6) PLACER {P}{LACE}{r..eR}
25 Author leads to some atrociously killer instincts (4) SAKI Acrostic

GRID