50% of students take a key part in it (5)
I believe the answer is:
study
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'50% of students take a key part in' is the wordplay.
'50% of' indicates taking half.
'take' is a charade indicator (letters next to each other).
'a key part in' becomes 'y' (I can't justify this - if you can you should give a lot more credence to this answer).
'students' with half the letters taken is 'stud'.
'stud'+'y'='STUDY'
Can you help me to learn more?
(Other definitions for study that I've seen before include "Scrutinise" , "Room for work/reading" , "Dusty (anag.)" , "Work-room" , "Investigate" .)