Many hounds sat barking (9)
I believe the answer is:
thousands
'many' is the definition.
(thesaurus)
'hounds sat barking' is the wordplay.
'barking' is an anagram indicator (UK slang for mad).
'hounds'+'sat'='houndssat'
'houndssat' is an anagram of 'THOUSANDS'.
(Other definitions for thousands that I've seen before include "Dan shouts out at many hundreds" , "1000s" , "large sum?" , "Large numbers" , "a lot" .)