To applaud section retreating is nonsense (8)
I believe the answer is:
claptrap
'is nonsense' is the definition.
(thesaurus)
'to applaud section retreating' is the wordplay.
'to applaud' becomes 'clap' ('clap' can be a synonym of 'applaud').
'section' becomes 'part' ('part' can be a synonym of 'section'**).
'retreating' says the letters should be written in reverse.
'part' written backwards gives 'trap'.
'clap'+'trap'='CLAPTRAP'
(Other definitions for claptrap that I've seen before include "Pretentious talk or nonsense" , "Absurd or nonsensical talk or ideas" , "poppycock" , "moonshine" , "Rhyme" .)