Expert adviser is relentless about fuel regularly being wasted (10)
I believe the answer is:
consultant
'expert adviser' is the definition.
'relentless about fuel regularly being wasted' is the wordplay.
'relentless' becomes 'constant'.
'about' means one lot of letters goes inside another.
'regularly being wasted' indicates alternate letters (letters removed at regular intervals).
The alternate letters of 'fuel' are 'ul'.
'constant' going around 'ul' is 'CONSULTANT'.
'is' is the link.
(Other definitions for consultant that I've seen before include "Senior hospital doctor" , "Specialist" , "See 1 across" , "Adviser" .)