Solution to 5D 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 In other words, a moist European plant (4,2,1,4) LOVE IN A MIST {0} in {A}{MIST} = {A MOIST}
10 Master control, block capital (9) AMSTERDAM {MASTER*}{DAM}
11 Union member comfortable in prison (9) BRIDEWELL {BRIDE}{WELL}
12 Dig into some strudel, very tasty (5) DELVE [T]
13 Approval to review communist reformer (7) AMENDER {AMEN}{RED<=}
15 Rate part of movie promotion (4) RAIL tRAILer
20 Figure out and log in to the city (7) COLOGNE {CO{LOG}NE}
23 Vessel returning from Muscat, roaming all over (5) AORTA [T<=]
24 Clubs, perhaps, arranging mini tours (5,4) MINOR SUIT*
26 Mental illness — pity it goes along with 19 (9) PYROMANIA {PitY}{ROMANIA}
28 Alcoholic very tense (3) SOT {SO}{T}
DOWN
1 Capital stay, one supports protein rich food (4,4) LIMA BEAN {LIMA}{B{E}AN} (Correction - {LIMA}{BE}{AN} - See comments)
3 Paper goes topless, it's a problem (5) ISSUE tISSUE
4 A friend's hurt, being exposed as a greenhorn (7) AMATEUR {A}{MATE}{hURt}
5 Small-town ceramic, perhaps to be removed (7) I?S?L?R (Addendum - INSULAR INSULAtoR - See comments)
6 Sing about doctor made exercise equipment (9) TREADMILL {TR{MADE*}ILL}
7 Doubled increase of fraud (6) FIDDLE {DD} in {FILE}
8 Moderate check on Pound (6) DAMPEN {DAM}{PEN}
14 Truly assumed charge and gave a speech (9) DECLAIMED {CLAIM} {inDEED}
17 It accommodates a subject not belonging to any country (8) SENTENCE [DD?]
19 Broadcast over our PM shortly coming up in country (7) ROMANIA {AI{NAMO}R}<=
21 Bones formed by vehicle discharge (6) CARPUS {CAR}{PUS}
22 Fungal attack on door frame, right from below (3-3) DRY-ROT {DooR}{TORY<=}
25 Place to rest, very big in Erode (5) ROOST {RO{OS}T}
Reference List
Nothing much = FA(Fanny Adams), Right = R, Standard = S, Tense = T, Very big = OS





