Poem by Keats ultimately accessible via web (6)
I believe the answer is:
sonnet
'poem' is the definition.
(type of poem)
'keats ultimately accessible via web' is the wordplay.
I cannot really understand how this works, but
'web' could be 'net' (synonyms) and 'net' is found in the answer.
The remaining letters 'son' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'by' is the link.
Can you help me to learn more?
(Other definitions for sonnet that I've seen before include "14-line poem" , "Fourteen line poem" , "Tonnes (anag.)" , "Fourteen-line poem" , "Shakespearean poem" .)