Author shows computer expert diving into river (9)
I believe the answer is:
thackeray
'author' is the definition.
(I've seen this before)
'computer expert diving into river' is the wordplay.
'computer expert' becomes 'hacker' (I've seen this before).
'diving into' is an insertion indicator.
'river' becomes 'tay' (Scottish river).
'hacker' put within 'tay' is 'THACKERAY'.
'shows' acts as a link.
(Other definitions for thackeray that I've seen before include "Vanity Fair author" , "novel creator" , "Pendennis author" , "William Makepeace --, English novelist" , "W.M. -, Eng. novelist" .)