Desmond initially knowing best type of computer (7)
I believe the answer is:
desktop
'type of computer' is the definition.
(desktop computer)
'desmond initially knowing best' is the wordplay.
'desmond' becomes 'des' (short for Desmond).
'initially' suggests taking the first letters.
'best' becomes 'top' (top can mean the very best).
The first letter of 'knowing' is 'k'.
'des'+'k'+'top'='DESKTOP'
(Other definitions for desktop that I've seen before include "Kind of PC (for publishing?)" , "Working surface, computer screen area" , "often supports a mass of papers?" , "Publishers'" , "Main working area of PC" .)