Contents of cylinder, viscous fuel (4)
I believe the answer is:
derv
'fuel' is the definition.
(I know that derv is a type of diesel fuel)
'contents of cylinder viscous' is the wordplay.
'contents of' says the answer is hidden in the clue.
'DERV' is hidden within 'cylinder viscous'.
(Other definitions for derv that I've seen before include "Lorry fuel" , "Fuel for lorries" , "Vehicle fuel oil" , "Vehicle diesel-oil" , "Diesel engine fuel oil" .)