Devise a scheme that's ordinary without one (4)
I believe the answer is:
plan
'devise a scheme' is the definition.
'plan' can be an answer for 'devise' (synonyms). I am not sure about the 'a scheme' bit.
'ordinary without one' is the wordplay.
'ordinary' becomes 'plain' (I've seen this in another clue).
'without' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'plain' with 'i' removed is 'PLAN'.
'that's' is the link.
Can you help me to learn more?
(Other definitions for plan that I've seen before include "Layout drawing" , "Map; intention" , "Project" , "Off the right direction" , "Detailed proposal" .)