Apt to forget half of list in manifest (9)
I believe the answer is:
oblivious
'apt to forget' is the definition.
(I've seen this before)
'half of list in manifest' is the wordplay.
'half of' indicates taking half.
'in' means one lot of letters goes inside another.
'manifest' becomes 'obvious' (similar in meaning).
'list' with half the letters taken is 'li'.
'li' put inside 'obvious' is 'OBLIVIOUS'.
(Other definitions for oblivious that I've seen before include "like Lethe?" , "Not noticing" , "Absent-mindedly unaware" , "totally in the dark?" , "Blind (to)" .)