Rubbish suggestion mostly thrown up in computer program (6)
I believe the answer is:
editor
'computer program' is the definition.
(editor is a kind of computer program)
'rubbish suggestion mostly thrown up' is the wordplay.
'rubbish' becomes 'rot' (both can mean nonsense).
'suggestion mostly' becomes 'ide' (I am not sure about this - if you are sure you should believe this answer much more).
'thrown up' says the letters should be written in reverse (in a down clue, letters go up).
'rot'+'ide'='rotide'
'rotide' reversed gives 'EDITOR'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for editor that I've seen before include "Newspaper executive" , "One who orders copy" , "Rag worker" , "Press boss" , "Reviser" .)