Clear but doesn't have one idea (4)
I believe the answer is:
plan
'idea' is the definition.
(plan is a kind of idea)
'clear but doesn't have one' is the wordplay.
'clear' becomes 'plain' (synonyms).
'but doesn't have' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'plain' with 'i' taken out is 'PLAN'.
(Other definitions for plan that I've seen before include "Design; map" , "Schedule" , "Arrangement" , "Stratagem" , "Method laid out for accomplishing something" .)