Sump is to necessitate purchasing engine part (6)
I believe the answer is:
piston
'engine part' is the definition.
(I've seen this before)
'sump is to necessitate purchasing' is the wordplay.
'purchasing' says the answer is hidden in the clue.
'PISTON' can be found hidden inside 'sump is to necessitate'.
(Other definitions for piston that I've seen before include "Block sliding in hollow cylinder in machinery etc" , "Transmitter of motion from camshaft to valve" , "Trumpet part" , "Points about driving rod" , "A moving part of car engine" .)