Nut regularly failed to seize day? (6)
I believe the answer is:
almond
'nut' is the definition.
(almond is a type of nut)
'regularly failed to seize day?' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'to seize' means one lot of letters goes inside another.
'day?' becomes 'mon' (abbreviation for Monday).
The alternate letters of 'failed' are 'ald'.
'ald' going around 'mon' is 'ALMOND'.
(Other definitions for almond that I've seen before include "Oval-shaped edible seed" , "Kind of nut, makes nice icing on cake" , "Fruit for marzipan" , "Nut (tree)" , "Old man (anag)" .)