To summarise, harvest about a hundred (5)
I believe the answer is:
recap
'to summarise' is the definition.
(recapping is a kind of summarizing)
'harvest about a hundred' is the wordplay.
'harvest' becomes 'reap' (synonyms).
'about' means one lot of letters goes inside another.
'a hundred' becomes 'C' (Roman numeral).
'reap' enclosing 'c' is 'RECAP'.
(Other definitions for recap that I've seen before include "In brief, go over the main points again" , "Restate" , "Go over again with backward pacer" , "Go over study material again" , "Summarise (colloq.)" .)