You forget 50 per cent of ideas (4)
I believe the answer is:
thou
'you' is the definition.
(historical second person pronoun)
'50 per cent of ideas' is the wordplay.
'50 per cent of' means to take half the letters.
'ideas' becomes 'thoughts' ('thought' can be a synonym of 'idea').
'thoughts' with half the letters taken is 'THOU'.
'forget' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for thou that I've seen before include "old address" , "Old-fashioned pronoun" , "You, in olden times" , "Refers to the person addressed (archaic)" , "Archaic pronoun" .)