Sorry to start running on the wrong fuel (6)
I believe the answer is:
rueful
'sorry' is the definition.
(synonyms)
'start running on the wrong fuel' is the wordplay.
'start running on' becomes 'ru' (I can't justify this - if you can you should give a lot more credence to this answer).
'the wrong' indicates an anagram.
'fuel' with letters rearranged gives 'eful'.
'ru'+'eful'='RUEFUL'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for rueful that I've seen before include "Melancholy" , "Regretting" , "Apologetic" , "Contrite, with regrets" , "Full of regret" .)