Oil? Carful does, regularly (5)
I believe the answer is:
crude
'oil?' is the definition.
(crude is a kind of oil)
'carful does regularly' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'carful'+'does'='carfuldoes'
The alternating letters of 'carfuldoes' are 'CRUDE'.
(Other definitions for crude that I've seen before include "foul-mouthed?" , "Not refined, like oil" , "Petroleum" , "Not refined, oil say" , "Unrefined; makeshift" .)