Fail when beginning to talk nonsense (7)
I believe the answer is:
bombast
'nonsense' is the definition.
(thesaurus)
'fail when beginning to talk' is the wordplay.
'fail' becomes 'bomb' (synonyms).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'beginning to' indicates taking the first letters.
The initial letter of 'talk' is 't'.
'bomb'+'as'+'t'='BOMBAST'
(Other definitions for bombast that I've seen before include "hollow material" , "High-flown language" , "Pompous talk" , "Pompous language" , "Pompously inflated language" .)