Good for dieters, regularly needed in undoing feasts? (6)
I believe the answer is:
nonfat
'good for dieters' is the definition.
'regularly needed in undoing feasts?' is the wordplay.
'regularly needed in' indicates alternate letters.
'undoing'+'feasts'='undoingfeasts'
The alternate letters of 'undoingfeasts' are 'NONFAT'.
(Other definitions for nonfat that I've seen before include "Lacking grease" , "Like some low-calorie food" , "Grade of milk" , "Low calorie" .)