About half of score in many cases (5)
I believe the answer is:
often
'in many cases' is the definition.
(I've seen this before)
'about half of score' is the wordplay.
'about' becomes 'of'.
'half of score' becomes 'ten' (I can't justify this - if you can you should believe this answer much more).
'of'+'ten'='OFTEN'
Can you help me to learn more?
(Other definitions for often that I've seen before include "More than occasionally" , "Perhaps daily" , "Turn away" , "Frequent" , "Frequently, at short intervals" .)