Gun that's shown in front of clergyman (3)

I believe the answer is:

rev

I'm a little stuck... Click here to teach me more about this clue!

'clergyman' is the definition.
(short for reverend)

I cannot understand how the rest of the clue works.

(Other definitions for rev that I've seen before include "Speed up engine (colloq.)" , "Clergy title" , "Accelerate (abbrev.)" , "Reverend (abbr.)" , "In engine-room speed up" .)

I've seen this clue in The Times.

Want a hint initially instead of a full solution? Install my app