Unused baseball glove holds dictionary open (7)
I believe the answer is:
omitted
'unused' is the definition.
The answer and definition are not the same part of speech. However, adjectives and past participle verbs occasionally define each other.
'baseball glove holds dictionary open' is the wordplay.
'baseball glove' becomes 'mitt' (synonyms).
'holds' is an insertion indicator.
'dictionary open' becomes 'oed' (I am not sure about this - if you are sure you should believe this answer much more).
'mitt' put inside 'oed' is 'OMITTED'.
Can you help me to learn more?
(Other definitions for omitted that I've seen before include "Cardigan boy" , "not mentioned" , "forgotten" , "Dropped" , "Left out, probably forgot" .)