Hand in dictionary that's left out (7)
I believe the answer is:
omitted
'left out' is the definition.
(to omit something is to leave it out)
'hand in dictionary' is the wordplay.
'hand' becomes 'mitt' (synonyms).
'in' means one lot of letters goes inside another.
'dictionary' becomes 'OED' (Oxford English Dictionary).
'mitt' put within 'oed' is 'OMITTED'.
'that's' is the link.
(Other definitions for omitted that I've seen before include "Left out - neglected" , "Musical aptitude" , "Dropped" , "Left out, probably forgot" , "Failed" .)