I found this harder than usual – solving time 39 minutes.
* = anagram
ACROSS
1 CAR ACT (A CU) S Chieftain who fought the Romans in Britain Cu = copper
10 CON CORD
11 PR A LINE
12 BEDROOM SLIPPERS (posed problem, sir)*
13 APERIENT (pie aren’t)* laxative
15 P (soft – music) ADDED
17 DEB ATE (Greek goddess of mischief) ‘Little’ girl refers to the abbreviation, I think.
19 S P (first letters) ANGLED
22 BACHELOR OF MUSIC ‘score’ always suggests music
25 TOR (MEN) T
26 YOU WAIT U (upper class) “weight”
27 MISPRINTED (mind priest)*
DOWN
2 A GOG O
3 ABD OMEN (bad)*
4 TI (pp) LE
5 C HAM PLAIN De Champlain – explored Canada
6 STIPEND Cloth = clergy
7 SCAB BARD
8 ON EDGE Refers to the Wenlock Edge.
9 NESS Changing last letter of nest = home
14 IN THE NEWS (Ten is when)*
16 D (E) DICAT (E) (addict)* with e = drug (ecstacy) twice
18 BUCK RAM
20 PROC (laim) YON
21 LASCAR Hidden
22 Stephen POTTER who coined the term
23 BYTE “bite”
24 M (O) USE Computer mouse.