Tenner stupidly invested in IT and computer network (8)
I believe the answer is:
internet
'computer network' is the definition.
(I know that internet is a type of computer network)
'tenner stupidly invested in it' is the wordplay.
'stupidly' indicates an anagram (a stupid ordering of the letters).
'invested in' indicates putting letters inside.
'tenner' anagrammed gives 'nterne'.
'nterne' going into 'it' is 'INTERNET'.
'and' acts as a link.
(Other definitions for internet that I've seen before include "Electronic communication system" , "What could be offered at some cafes" , "Here, blogs" , "Information superhighway" , "Computer service" .)