The back half of dinosaur you and I saw is alongside gnome here (9)
I believe the answer is:
thesaurus
'saw is alongside gnome here' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'the back half of dinosaur you and i' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'back' becomes 'he' (I can't explain this - if you can you should give a lot more credence to this answer).
'half of' suggests halving the letters.
'you and i' becomes 'us' (the two of us).
'dinosaur' halved is 'saur'.
't'+'he'+'saur'+'us'='THESAURUS'
Can you help me to learn more?
(Other definitions for thesaurus that I've seen before include "Reference work" , "Word book" , "lists?" , "work we may refer to" , "Book of words with synonyms, antonyms etc" .)