Concept, but ahead of time (7)
I believe the answer is:
thought
'concept' is the definition.
(I have seen 'A concept' mean 'thought' so perhaps 'concept' could also mean 'thought')
'but ahead of time' is the wordplay.
I cannot quite understand how this works, but
'but' could be 'though' (I've seen this before) and 'though' is located in the answer.
'time' could be 't' (abbreviation) and 't' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for thought that I've seen before include "Reflection" , "Notion" , "care" , "Deliberated" , "just a trifle" .)