An easy Mudd this time around. 19D was the only clue that stumped me for a while.
Across
1. HOPPER – [c]HOPPER
4. SCHOONER – CH (companion) in SOONER (rather)
9. READER – DE (two low grades) in REAR (behind)
10. INDICTED – INDIC[a]TED (shown with A[ccusations] removed)
12. BIKE – hidden word
13. SANTA MARIA – TAMAR (river) + I (one) in SA (South Africa)
15. CONVERSATION -AT (a time) in CONVERSION (change)
18. ONOMATOPOEIA – ON (on) + OM (order) + AT (at) + OP (work) + {A (article) + IE (that is) + O (old)} backwards
21. THOMAS COOK – M (capital of Malawi) in anagram of CHAOS in TOOK (guided)
22. ERNE – reverse hidden word
24. BOOB TUBE – BOOB (mistake) + TUBE (public transport network)
25. JORDAN – double definition
26. ENGINEER – E (close to accomplice) + anagram of IN GREEN
27. MORSEL – S (last of fungus) in MOREL (mushroom)
Down
1. HARDBACK – HARD (alchoholic) + BACK (sponsor)
2. PLANKING – PLAN (design) + KING (piece on [chess] board)
3. EWER – [s]EWER
5. CONSTITUTION – anagram of IS NOT CUT INTO
6. OPIUM POPPY – cryptic definition
7. NATURE – TAN (shade) backwards + URE (river)
8. REDIAL – anagram of DERAIL
11. DANSE MACABRE – DAN (boy) + CAME (reached) backwards in SABRE (sword)
14. DEAN MARTIN – DEAN (clergyman) + MARTIN (bird)
16. HEBRIDES – cryptic definition (“he brides”)
17. PATERNAL – A (a) + TERN (bird) in PAL (friend)
19. STABLE – double definition with “string” meaning a collection of animals. This usage of “string” is a little obscure – which is fine – but the wording seems a little awkward.
20. SO LONG – SOLO (individual) + N[ottinghamshire] + G[lamorgan]
23. BOZO – BOZ (Dickens) + O (nothing). “Boz” was an early pen-name of Charles Dickens.