Guide to terrible scan (6)
I believe the answer is:
direct
'guide' is the definition.
(I know that guide is a more specific form of direct)
'terrible scan' is the wordplay.
'terrible' becomes 'dire' (synonyms).
'scan' becomes 'ct' (I can't explain this - if you can you should give a lot more credence to this answer).
'dire'+'ct'='DIRECT'
'to' is the link.
Can you help me to learn more?
(Other definitions for direct that I've seen before include "Organise; straight" , "Outspoken" , "Give travel guidance" , "Credit with being forthright" , "By the shortest route" .)