The reptilian writer's resource (9)
I believe the answer is:
thesaurus
'writer's resource' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'the reptilian' is the wordplay.
'reptilian' becomes 'saurus' (I can't explain this - if you can you should believe this answer much more).
'the'+'saurus'='THESAURUS'
Can you help me to learn more?
(Other definitions for thesaurus that I've seen before include "work we may refer to" , "Wordy gathering" , "Word book" , "storehouse" , "Book of words with synonyms, antonyms etc" .)