Frequently seen back in internet forum (5)
I believe the answer is:
often
'frequently' is the definition.
(synonyms)
'back in internet forum' is the wordplay.
'back' says the letters should be written backwards.
'in' indicates a hidden word.
'netfo' can be found hidden inside 'internet forum'.
'netfo' reversed gives 'OFTEN'.
'seen' acts as a link.
(Other definitions for often that I've seen before include "Frequently, many times" , "Commonly" , "Turn away" , "A great deal" , "Repeatedly with regularity" .)