Thought I had to take notes (4)
I believe the answer is:
idea
'thought' is the definition.
(I know that thought can be written as idea)
'i had to take notes' is the wordplay.
'i had' becomes 'i'd' (contraction).
'to take' says to put letters next to each other.
'notes' becomes 'ea' ().
'id'+'ea'='IDEA'
(Other definitions for idea that I've seen before include "intellectual product" , "Thought, conception" , "Brainwave" , "Aim" , "Thought, belief" .)