Fast pulse? Reduce by up to 51 (4)

I believe the answer is:

lent

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

'fast' is the definition.
(Lent is a period of fasting in Christianity)

I don't understand the remainder of the clue.

(Other definitions for lent that I've seen before include "Loaned; a fast" , "Forty days of penance" , "Forty days of fasting" , "Pre-Easter period" , "Christian period of abstinence" .)

I've seen this clue in The Mirror.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.