Sit, eating out, being prompt (9)
I believe the answer is:
instigate
'prompt' is the definition.
(I know that prompt can be written as instigate)
'sit eating out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'sit'+'eating'='siteating'
'siteating' with letters rearranged gives 'INSTIGATE'.
'being' is the link.
(Other definitions for instigate that I've seen before include "Start" , "Provoke" , "Bring about by incitement - its eating (anag)" , "Prompt" , "Set off or stir up" .)