Fail when beginning to talk - it's hot air (7)
I believe the answer is:
bombast
'hot air' 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'
'it's' is the link.
(Other definitions for bombast that I've seen before include "pretentious speech" , "Pomposity" , "hollow material" , "High-sounding language with little meaning" , "Pompously inflated language" .)