Hand in dictionary that is left out (7)
I believe the answer is:
omitted
'is left out' is the definition.
(to omit something is to leave it out)
'hand in dictionary' is the wordplay.
'hand' becomes 'mitt' (synonyms).
'in' indicates putting letters inside.
'dictionary' becomes 'OED' (Oxford English Dictionary).
'mitt' placed into 'oed' is 'OMITTED'.
'that' is the link.
(Other definitions for omitted that I've seen before include "forgotten" , "Disregarded" , "Missed out" , "Musical aptitude" , "not mentioned" .)