Fansite chiefly, but not entirely, for computer enthusiast (6)
I believe the answer is:
techie
'computer enthusiast' is the definition.
(someone interested in technology)
'fansite chiefly but not entirely' is the wordplay.
'not entirely' says the answer is hidden in the clue.
'TECHIE' is hidden in the letters of 'fansite chiefly but'.
'for' acts as a link.
(Other definitions for techie that I've seen before include "Expert in some practical science" , "computer expert" , "PC expert" , "IT whizz" .)