Unhappy with oil, initially changing fuel (6)
I believe the answer is:
woeful
'unhappy' is the definition.
(similar in meaning)
'with oil initially changing fuel' is the wordplay.
'initially' says to take the initial letters.
'changing' indicates anagramming the letters.
The first letters of 'with oil' is 'wo'.
'fuel' anagrammed gives 'eful'.
'wo'+'eful'='WOEFUL'
(Other definitions for woeful that I've seen before include "Deplorable or wretched" , "We foul, being sad and deplorable" , "Miserable" , "'Wilful waste makes ...... want'" , "Pathetic" .)