Had witnessed and heard the flare-up (4)
I believe the answer is:
seen
'had witnessed' is the definition.
I can't judge whether this definition defines the answer.
'and heard the flare-up' is the wordplay.
'and' becomes 'n' (common abbreviation for 'and').
'heard the flare' becomes 'ees' (I can't justify this - if you can you should believe this answer much more).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'n'+'ees'='nees'
'nees' reversed gives 'SEEN'.
Can you help me to learn more?
(Other definitions for seen that I've seen before include "Sighted" , "Observed or percieved" , "Espied" , "watched" , "Noticed" .)