Old fuel may make you miserable (7)
I believe the answer is:
doleful
'miserable' is the definition.
(I've seen this before)
'old fuel may make' is the wordplay.
'may make' indicates anagramming the letters (I've seen this in other clues).
'old'+'fuel'='oldfuel'
'oldfuel' with letters rearranged gives 'DOLEFUL'.
'you' is the link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for doleful that I've seen before include "down in the dumps" , "Melancholic" , "Cheerless" , "Causing grief; sorrowful" , "Sad" .)