Hand in dictionary -- it's left out (7)
I believe the answer is:
omitted
'left out' is the definition.
('omit' can be a synonym of 'leave out')
'hand in dictionary' is the wordplay.
'hand' becomes 'mitt' ('mitt' can be a synonym of 'hand').
'in' means one lot of letters goes inside another.
'dictionary' becomes 'oed' (Oxford English Dictionary).
'mitt' going into 'oed' is 'OMITTED'.
'it's' acts as a link.
(Other definitions for omitted that I've seen before include "Left out, probably forgot" , "Hairdressing" , "Leave undone or leave out" , "Musical aptitude" , "Skipped" .)