Fire up, as an engine (3)
I believe the answer is:
rev
'fire up as an engine' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rev that I've seen before include "Run (engine) quickly" , "vicar?" , "Reverend (abbr.)" , "serviceman?" , "In engine-room speed up" .)