Thought I had to start early (4)
I believe the answer is:
idea
'thought' is the definition.
(I know that thought can be written as idea)
'i had to start early' is the wordplay.
'i had' becomes 'I'd' (I'd).
'to start early' becomes 'ea' (I can't explain this - if you can you should give a lot more credence to this answer).
'id'+'ea'='IDEA'
Can you help me to learn more?
(Other definitions for idea that I've seen before include "Content of cognition" , "Flash of inspiration?" , "Notion, image" , "Suggestion" , "A suspicion of" .)