Relaxing by sea, fuel getting wasted (7)
I believe the answer is:
easeful
'relaxing' is the definition.
(I've seen this in another clue)
'by sea fuel getting wasted' is the wordplay.
'by' means one lot of letters go next to another.
'getting wasted' indicates anagramming the letters.
'sea' put next to 'fuel' is 'seafuel'.
'seafuel' anagrammed gives 'EASEFUL'.
(Other definitions for easeful that I've seen before include "Sea fuel (anag)" , "Relaxing" , "giving relief" , "Tranquil" , "Calm" .)