Quick to decode unknown (5)
I believe the answer is:
ready
'quick' is the definition.
(similar in meaning)
'decode unknown' is the wordplay.
I cannot really understand how this works, but
'unknown' could be 'y' (algebra) and 'y' is located in the answer.
The remaining letters 'read' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'to' is the link.
Can you help me to learn more?
(Other definitions for ready that I've seen before include "Prepare money" , "Poised for action" , "Suitably prepared" , "rough companion" , "Quick" .)