Earth footage beginning to explain rare event (7)
I believe the answer is:
eclipse
'rare event' is the definition.
'earth footage beginning to explain' is the wordplay.
'earth' becomes 'e'.
'footage' becomes 'clips'.
'beginning to' suggests taking the first letters.
The initial letter of 'explain' is 'e'.
'e'+'clips'+'e'='ECLIPSE'
(Other definitions for eclipse that I've seen before include "Heavenly event; drake's time of dull plumage" , "Blot out" , "Obscure, as moon may the sun" , "Throw into obscurity, perhaps the sun" , "Throw into shade, like a heavenly body" .)