Thought I had an early start (4)
I believe the answer is:
idea
'thought' is the definition.
(I know that thought can be written as idea)
'i had an early start' is the wordplay.
'i had' becomes 'i'd' (I'd).
'an early start' becomes 'ea' (I am not sure about this - if you are sure 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 "Thought, belief" , "Suggestion" , "A suspicion of" , "Notion in the mind" , "Aim" .)