Big Herb is relaxing in bunk (9)
I believe the answer is:
gibberish
'bunk' is the definition.
(thesaurus)
'big herb is relaxing' is the wordplay.
'relaxing' indicates anagramming the letters.
'big'+'herb'+'is'='bigherbis'
'bigherbis' anagrammed gives 'GIBBERISH'.
'in' is the link.
(Other definitions for gibberish that I've seen before include "Tosh" , "Nonsensical talk" , "moonshine" , "Meaningless talk" , "Drivel" .)