Rents fuel out - that's bitter (9)
I believe the answer is:
resentful
'that's bitter' is the definition.
(synonyms)
'rents fuel out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'rents'+'fuel'='rentsfuel'
'rentsfuel' anagrammed gives 'RESENTFUL'.
(Other definitions for resentful that I've seen before include "feeling sore" , "Feeling hostile because of injury" , "Taking ill" , "Feeling indignation" , "Bitter" .)