Drop left in engine's oil reservoir (5)
I believe the answer is:
slump
'drop' is the definition.
(slumping is a kind of dropping)
'left in engine's oil reservoir' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'engine's oil reservoir' becomes 'sump' (I've seen this before).
'l' put inside 'sump' is 'SLUMP'.
(Other definitions for slump that I've seen before include "Sudden market drop" , "Sudden decline" , "Commercial decline" , "Fall heavily and limply" , "Heavy fall" .)