Do this to about a hundred and you have about fifty (5)
I believe the answer is:
halve
'do this to about a hundred and you' is the definition.
The definition suggests a singular noun but the answer is not.
'have about fifty' is the wordplay.
'about' is an insertion indicator.
'fifty' becomes 'L' (Roman numeral).
'have' enclosing 'l' is 'HALVE'.
Can you help me to learn more?
(Other definitions for halve that I've seen before include "Cut down substantially" , "cut by fifty percent" , "Divide fifty-fifty" , "Reduce by a factor of two" , "Bisect" .)