Thought of model endlessly (4)
I believe the answer is:
idea
'thought' is the definition.
(I know that thought can be written as idea)
'model endlessly' is the wordplay.
'model' becomes 'ideal' (ideal is a kind of model).
'endlessly' means to remove the last letter.
'ideal' with its final letter taken off is 'IDEA'.
'of' acts as a link.
(Other definitions for idea that I've seen before include "Brainwave" , "Content of cognition" , "Proposal" , "Plato's Form is" , "Flash of inspiration?" .)