People like Thomas Cook used to blather vacuously (8)
I believe the answer is:
doubters
'people' is the definition.
(doubter is a kind of person)
'cook used to blather vacuously' is the wordplay.
'cook' indicates anagramming the letters.
'vacuously' says to hollow out the word (remove centre letters).
'blather' with its middle removed is 'br'.
'used'+'to'+'br'='usedtobr'
'usedtobr' with letters rearranged gives 'DOUBTERS'.
'like thomas' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Another definition for doubters that I've seen is " They are suspicious".)