Tragic day briefly captured in very old film (5)
I believe the answer is:
video
'film' is the definition.
(I've seen this before)
'tragic day briefly captured in very old' is the wordplay.
'tragic day' becomes 'ides' (Julius Caesar was killed on the Ides of March).
'briefly' means to remove the last letter.
'captured in' indicates putting letters inside.
'very' becomes 'v' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ides' with its final letter removed is 'ide'.
'v'+'o'='vo'
'ide' put into 'vo' is 'VIDEO'.
(Other definitions for video that I've seen before include "Recorded film cassette" , "Capture on film" , "Visible part of a recording" , "Recorded visual image" , "(Tape for) recorder of film etc" .)