* Theme words (not further defined) are rivers; one river has to be added to the wordplay in 30 clues; two are needed in three more clues; and four are solutions.
Across
1* boiled over DOVE in BOILER
6 amman (Viet)NAM MA (rev) [alternative spelling of ‘amtman’]
9 Friars/friars double def [Blackfriars & patch on printed page]
15* dribble D(issolve) + RIBBLE
16 Flora double def
17* spoliated PO in DETAILS (rev)
18** campanile P(eninsul)A in CAM/NILE
19* testudo (t)UDO(r) after TEST
20* Nairn diN<AIR>Ner
21* saxe S(piraea) + AXE
23* underwent U(l)N(a) + DERWENT
24* taffeta TAFF + ETA [Greek letter]
26 early years RARELY EASY (anag)
28 stops SPOTS (rev)
30 heyday HEY (country dance) + DAY
33* Savonarola AVON in ’S/A/ROLA(g)
36* Unilever NILE in REVU(e) (rev)
37 bad B(ritish)/AD(vert)
40 nis (a)N(t)I(c)S
41* remained MAIN in REED [Robert & Oliver]
42* St Lawrence LECTERN WAS (anag)
43 re-echo R(oyal)E(ngineers) + EC (City of London) + HO(use)
44* Rhone roan (hom)
45 on schedule CLUES HONED (anag)
50 trimmed double def
52 butterfat BUTT/AFTER(anag)
54* exes EXE + S(hilling)
56 emeer agREEMEnt (hidden rev)
57 monthly H(orticultura)L in MONTY (Don)
58* sommelier SOMME + LIER
60 detective DE(f/T)FECTIVE
61* Roach/roach double def
62 Russian RUSSIAN (roulette)
63* Mearns EARN in MS (manuscript)
64* esker ESK + (th)E/R(iver)
65 grindstone DINNERS GOT (anag)
Down
1 bodice BOD/ICE
2 idiom I<DI>OM [Isle of Man]
3 embracery EM<BRA/C(aught)>ERY
4 one minute ONE(a) + MINUTE
5 Enfield (k)E(e)N (o)F(f)I(c)E(r) L(e)D [Royal Enfield bikes]
7* means testing TEST in MEAN SING
8 assault a salt (hom)
10 Reign of Terror rain oft error (hom)
11* Astaire (b)A(s)S(e)T + AIRE
12* sedentary EDEN in RY(railway)/STA(tion) (rev)
13** posture ST(reet) in PO/URE
14* so forth SO(-so) + FORTH
22 arriviste A STRIVER I (anag &Lit)
25* hems in EMS in HIN(d)
27 Raleigh rally (hom) [(Sir Walter Raleigh]
29* peloton LOT in PEON
31* debenture (blanke)T in DEBEN/URE
32 uvea (l)U(v)V(i)E + A
34* North American THAME in CORN IRAN (anag)
35 Rome RO(ugh ti)ME
38* deepen DEE + PEN (writer/bird)
39* Kenneth Horne KENNET + HH (very hard) + ORNE (French department)
43* Rotterdam OTTER in R(ight)/DAM
46* catamaran TAMAR in CAAN [James, star in Misery (1990)]
47* elevenses LEVEN in E/S E/S
48* à demain (h)ADE(s) + MAIN
49* New Year WYE in NEAR
51 iced tea ICE/DATE(anag)
52* banners BANN + ’ERS
53 Fischer FI<SCH(ool)>ER(y) [Bobby Fischer]
55* trente TRENT + (on)E [30 in French]
59 Inigo IN I GO!