Regularly alternate fast (4)

I believe the answer is:

lent

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

'regularly alternate' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'alternate' are 'LENT'.

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

I've seen this clue in The Guardian.

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