Idea that is however ahead of time (7)
I believe the answer is:
thought
'idea' is the definition.
('thought' can be a synonym of 'idea')
'however ahead of time' is the wordplay.
'however' becomes 'though' ('though' can be a synonym of 'however').
'ahead of time' becomes 't' (I've seen this in other clues).
'though'+'t'='THOUGHT'
'that is' is the link.
(Other definitions for thought that I've seen before include "Reflection" , "Performed brain operations" , "Deliberation" , "Supposed" , "Content of cognition" .)