A concept, however ahead of time (7)
I believe the answer is:
thought
'a concept' is the definition.
(I have seen 'A concept' mean 'thought' so perhaps 'concept' could also mean 'thought')
'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'
(Other definitions for thought that I've seen before include "care" , "Idea, concept" , "That's an idea" , "Thinking process" , "Cogitated" .)