Obvious - doesn't have one idea (4)
I believe the answer is:
plan
'idea' is the definition.
(plan is a kind of idea)
'obvious doesn't have one' is the wordplay.
'obvious' becomes 'plain' (similar in meaning).
'doesn't have' is a deletion indicator.
'one' becomes 'i' (Roman numeral).
'plain' with 'i' removed is 'PLAN'.
(Other definitions for plan that I've seen before include "Off the right direction" , "Intention or decision about what one is going to do" , "Arrangement" , "Map; intention" , "Design; map" .)