Writer rejected investing in computers as foolish (5)
I believe the answer is:
inept
'foolish' is the definition.
(I've seen this before)
'writer rejected investing in computers' is the wordplay.
'writer' becomes 'pen' (something which writes).
'rejected' says the letters should be written in reverse.
'investing in' indicates putting letters inside.
'computers' becomes 'IT' (information technology).
'pen' reversed gives 'nep'.
'nep' placed into 'it' is 'INEPT'.
'as' acts as a link.
(Other definitions for inept that I've seen before include "Cack-handed" , "Futile" , "Clumsy, bungling" , "Incompetent and ineffectual" , "Awkward, clumsy" .)