Joy swept up contents of fossil bed (5)
I believe the answer is:
bliss
'joy' is the definition.
(thesaurus)
'swept up contents of fossil bed' is the wordplay.
'swept up' shows that the letters should be reversed in order (in a down clue, letters go up).
'contents of' indicates the answer is hidden within the clue.
'ssilb' is hidden within 'fossil bed'.
'ssilb' reversed gives 'BLISS'.
(Other definitions for bliss that I've seen before include "Great happiness" , "Serene joy" , "of composing?" , "Beatitude" , "Great joy" .)