Online commenter's weight has someone laughing about it (9)
I believe the answer is:
twitterer
'online commenter's weight' is the definition.
I can't judge whether this definition defines the answer.
'someone laughing about it' is the wordplay.
I cannot quite see how this works, but
'someone' could be 'wit' (wit is a kind of someone) and 'wit' is located in the answer.
'about' could be 're' ('re' means regarding or about) and 're' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
The remaining letters 'ter' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'has' acts as a link.
Can you help me to learn more?
(Other definitions for twitterer that I've seen before include "Chirper" , "One talking rapidly or nervously" , "Trivial chatter; bird" .)