Allow new entry, fast (4)
I believe the answer is:
lent
'fast' is the definition.
(I've seen this before)
'allow new entry' is the wordplay.
'allow' becomes 'let' ('let' can be a synonym of 'allow').
'new' becomes 'n'.
'entry' means one lot of letters goes inside another.
'let' enclosing 'n' is 'LENT'.
(Other definitions for lent that I've seen before include "Period of penance" , "Loaned; a fast" , "Pre-Easter period" , "From Ash Wednesday to Holy Saturday" , "Period from Ash Wednesday to Easter" .)