The buttocks let out hot air (7)
I believe the answer is:
prattle
'hot air' is the definition.
(thesaurus)
'the buttocks let out' is the wordplay.
'the buttocks' becomes 'prat' ('prat' can be a synonym of 'buttocks'**).
'out' indicates an anagram (out can mean wrong or inaccurate).
'let' is an anagram of 'tle'.
'prat'+'tle'='PRATTLE'
(Other definitions for prattle that I've seen before include "Talk emptily" , "Childish talk" , "Be vapidly garrulous" , "Talk inanely" , "Inconsequential chatter" .)