Sing a song about fuel? (3)
I believe the answer is:
gas
'fuel?' is the definition.
(gas is a kind of fuel)
'sing a song about' is the wordplay.
'about' indicates a hidden word (the clue goes around or about the answer).
'GAS' is hidden within 'sing a song'.
(Other definitions for gas that I've seen before include "Form of fossil fuel" , "Piped fuel" , "natural resource" , "Possibly noble" , "16 (US)" .)