Gather about a hundred facts in summary (5)
I believe the answer is:
recap
'facts in summary' is the definition.
'recap' can be an answer for 'summary' (recap is a kind of summary). I'm unsure of the 'facts in' bit.
'gather about a hundred' is the wordplay.
'gather' becomes 'reap' (reaping is a kind of gathering).
'about' is an insertion indicator.
'a hundred' becomes 'c' (Roman numeral).
'reap' placed around 'c' is 'RECAP'.
Can you help me to learn more?
(Other definitions for recap that I've seen before include "Repeat of main points" , "Summarise, run over" , "Sum up what has been said" , "In summary, state again" , "Go over old ground" .)