Part of sombre hotel set up by Rosemary (4)
I believe the answer is:
herb
'rosemary' is the definition.
(thesaurus)
'part of sombre hotel set up' is the wordplay.
'part of' indicates a hidden word.
'set up' is a reversal indicator (in a down clue, letters go up).
'breh' is hidden within 'sombre hotel'.
'breh' backwards is 'HERB'.
'by' acts as a link.
(Other definitions for herb that I've seen before include "bay perhaps" , "Marjoram, say" , "ingredient for recipe?" , "Medicinal or culinary plant" , "Aromatic plant used in cookery" .)