Relaxing each second, wasting fuel (7)
I believe the answer is:
easeful
'relaxing' is the definition.
(I've seen this before)
'each second wasting fuel' is the wordplay.
'each' becomes 'ea' (abbreviation).
'second' becomes 's' (common abbreviation).
'wasting' indicates anagramming the letters (waste can mean to destroy).
'fuel' with letters rearranged gives 'eful'.
'ea'+'s'+'eful'='EASEFUL'
(Other definitions for easeful that I've seen before include "bringing relief" , "Sea fuel (anag)" , "Calm" , "Relaxing" , "Providing comfort" .)