Oil part in silent power turning over (9)
I believe the answer is:
petroleum
'oil' is the definition.
(I know that oil can be written as petroleum)
'part in silent power turning over' is the wordplay.
'part' becomes 'role' (eg a theatrical role).
'in' indicates putting letters inside.
'silent' becomes 'mute' (synonyms).
'power' becomes 'p'.
'turning over' shows that the letters should be reversed in order.
'mute'+'p'='mutep'
'mutep' in reverse letter order is 'petum'.
'role' going within 'petum' is 'PETROLEUM'.
(Other definitions for petroleum that I've seen before include "Hydrocarbon fuel" , "Hydrocarbon oil" , "Fossil fuel" , "Oil well product" , "It helps in getting things moving" .)