Say, rather badly: 'Bread vs rocks' (7)
I believe the answer is:
adverbs
'say rather badly' is the definition.
('rather' and 'badly' are adverbs)
'bread vs rocks' is the wordplay.
'rocks' indicates an anagram.
'bread'+'vs'='breadvs'
'breadvs' with letters rearranged gives 'ADVERBS'.
(Other definitions for adverbs that I've seen before include "Parts of speech" , "quickly, perhaps" , "Modifying words" , "They qualify or modify other words" .)