Chat show's opening minute promising much - but will never deliver (5,4)
I believe the answer is:
small talk
'chat' is the definition.
(I know that small talk is a type of chat)
'show's opening minute promising much but will never deliver' is the wordplay.
'opening' says to take the initial letters.
'minute' becomes 'm' (abbreviation).
'promising much but will never deliver' becomes 'all talk' (as in being 'all talk').
The first letter of 'shows' is 's'.
's'+'m'+'alltalk'='SMALL-TALK'
(Other definitions for small talk that I've seen before include "Social conversation on unimportant subjects" , "Social chat on unimportant matters" , "casual conversation" , "chattering" , "Polite conversation on trivial matters" .)