ACROSS
1 - Ice develops mysteriously on the old bicycles (11) - VELOCIPEDES*
9 - They're in conversation about a possessive adjective (5) - THEIR (~they're)
10 - Creative army engineer softly moves a step forward for the periodical (9) - NEWSPAPER {NEW}{S(+p)PA(-p)PER}
11 - Mall's hoopla zapped partially (5) - PLAZA [T]
12 - Strange grid in grill, something destined for Bombay High? (8,3) - DRILLING RIG*
13 - Revise what may keep a piece of software current (6) - UPDATE [DD]
14 - Pleased to have left the English out of the agreement (7) - COMPACT COMPlACenT
18 - Some newspaper staff get gold shillings after turning the trend around (7) - EDITORS {EDIT<-}{OR}{S}
20 - Reel on seeing Switzerland take over Padova's primary train (6) - SCHOOL {S(-p+ch}CHOOL}
24 - Famous East German and Pole found participating in a noisy mock serenade (4-7) - BEST-SELLING {B{EST}-{S}ELLING}
26 - Old city retreat sends back the gibbon to the country (5) - RURAL {RU<-}{RAL}
27 - Guard at the Tower of London, a non-vegetarian? (9) - BEEFEATER [DD]
29 - Realistic about where things in the air go due to gravity? (4-2-5) - DOWN-TO-EARTH [DD]
DOWN
1 - Metal found in dam Na'vi built to hide uranium (8) - VANADIUM {VANADI{U}M*}
2 - Base nature of the journalist following the poor idea (3-6) - LOW-MINDED {LOW}-{MIND}{ED}
3 - Company announcing infamous Judean governor as a flight crew member (7) - COPILOT {CO}{PILOT}(~pilate}
4 - Drop by with the student's missing ribbed cotton fabric (3,2) - POP IN {POPl IN}
5 - Negatives may be developed here (8) - DARKROOM [E]
6 - Discontinue rent for resort (7) - STOPGAP {STOP}{GAP}
7 - Secure the beer sent up for bottling in Belarus (5) - BELAY {B{ELA<-}Y}
8 - Daylight, clearly visible? (5) - BROAD [CD]
15 - Singer's awful rhetorics (9) - CHORISTER*
16 - Show off entertaining sailor's second-grade weapon (8) - CROSSBOW {CRO{SS}{B}{O}W} 'O' for ordinary grade?
19 - Rag, a bit old, is shredded (7) - TABLOID*
21 - Set lifted for packing by the old king in the seminary (7) - COLLEGE {COL{LEG<-}E}
22 - Impress Mark (5) - PRINT [DD]
23 - Mill in Switzerland embracing detailed plan (5) - CRUSH {C{RUSe}H}
25 - The Polish keep the Norse leader out for a short time in the film (5) - SHEET {SHEEn}{T}