Series of online messages is extremely tough to browse through (6)
I believe the answer is:
thread
'series of online messages' is the definition.
'thread' can be an answer for 'series' (thesaurus). I'm not certain of the 'of online messages' bit.
'extremely tough to browse through' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'to browse through' becomes 'read' (I am not sure about this - if you are sure you should believe this answer much more).
'tough' with its centre taken out is 'th'.
'th'+'read'='THREAD'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for thread that I've seen before include "Hatred (anag)" , "Fine cord of twisted fibres" , "Fine cord for sewing" , "The spiral ridge of a screw" , "clothes casually cut" .)