Now and again, film aunt eating sponge tart (4)
I believe the answer is:
flan
'tart' is the definition.
(type of tart)
'now and again film aunt' is the wordplay.
'now and again' indicates alternate letters (letters taken on a regular basis).
'film'+'aunt'='filmaunt'
The alternating letters of 'filmaunt' are 'FLAN'.
'eating sponge' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for flan that I've seen before include "Open pastry dessert" , "dish, sweet or savoury" , "Open pastry filled with savouries, fruit or custard" , "Open-topped pie" , "Open tart with a sweet or savoury filling" .)