One source of a writer's material (7)
'a writer's material' is the definition.
'inkwell' can be an answer for 'material' (thesaurus). I am unsure of the 'a writer's' bit.
'one source' is the wordplay.
'one' becomes 'ink' (I can't justify this - if you can you should believe this answer much more).
'source' becomes 'well' (well is a kind of source).
'of' is the link.
Can you help me to learn more?
(Other definitions for inkwell that I've seen before include "Desk reservoir" , "Small pot for holding writing fluid" , "Old school desk feature" , "Container sunk unto a desk of old" , "Reservoir for writing fluid" .)
(Other definitions for inkpot that I've seen before include "fluid container" , "Topper-up for old writer" , "that may assist writer" , "Source for old writer" , "Here perhaps blue fluid" .)