ACROSS
1 - Old book about how healthy blood's regenerated (9,4) - DOTHEBOYS HALL* Definition? Is there a book by this name?
9 - With honour, girl gets a goblet (7) - CHALICE {CH}{ALICE}
10 - A big noise is the troop leader, barbarian, reformed red (7) - THUNDER {T}{HUN}{DER*}
11 - A learned unknown trained by Macedonian hero (9) - ALEXANDER {A+LEARNED+X}*
12 - Passive and indolent miner trapped within (5) - INERT [T]
13 - The hardest thing in the body (6) - ENAMEL [CD]
14 - Get together and treat bishop with measles (8) - ASSEMBLE {B+MEASLES}*
17 - Congenial, akin to Lincoln having taken a drop of liquor (8) - LIKEABLE {LIKE}{AB{L}E}
19 - Strong jute fabric, non-white, is getting weaker … (6) - EBBING wEBBING
23 - … but silk fabric is right in a cloak (5) - CRAPE {C{R}APE}
25 - Vain, aloof rambling is useless (2,2,5) - OF NO AVAIL*
26 - More clamorous (7) - NOISIER [E]
27 - Clutching returned visa, girl is devious (7) - EVASIVE {EV{ASIV<-}E}
28 - 1000 little ones, they are part of a family (13) - GRANDCHILDREN {GRAND}{CHILDREN)
DOWN
1 - Stop batting, say? (7) - DECLARE [DD]
2 - Maruti maybe, or Maybach? (9) - TRADEMARK [CD]
3 - Leave the country without a trace of grief for Dubai, say (7) - EMIRATE EMIgRATE
4 - Exaggerate about the same in brief (6) - OVERDO {OVER}{DO}
5 - Posed with girl (English) to mock (8) - SATIRISE {SAT}{IRIS}{E}
6 - One, born suave, turns rude (7) - ABUSIVE {I+B+SUAVE}*
7 - Large advantage derived from projection (5) - LEDGE {L}{EDGE}
8 - Posh, better-designed lab item (7) - BURETTE {U+BETTER}*
15 - General officer on sailing ship gets a rough ride (9) - BRIGADIER {BRIG}{A}{DIER*} In the Indian Army he is still a field officer, General officers start from Major General onwards.
16 - Deep colour, not of aristocratic origin? (5-3) - BLOOD-RED [DD]
17 - Swindle in U.S. city, one caught using few words (7) - LACONIC {LA}{CON}{I}{C}
18 - Country to which a master carries cooked rice (7) - AMERICA {A}{M{ERIC*}A}
22 - The men she struggles to entangle! (6) - ENMESH*
24 - Imitating and losing time in recording (5) - APING tAPING