Thought setter would worry endlessly (4)
I believe the answer is:
idea
'thought' is the definition.
('idea' can be a synonym of 'thought')
'setter would worry endlessly' is the wordplay.
'setter' becomes 'i' (the person that set the crossword).
'would' becomes 'd' (I've seen this in other clues).
'worry' becomes 'eat' (eating is a kind of worrying).
'endlessly' means to remove the last letter.
'eat' with its last letter taken away is 'ea'.
'i'+'d'+'ea'='IDEA'
(Other definitions for idea that I've seen before include "Clue" , "Something imagined in the mind" , "Fancy" , "Thought; concept" , "It comes to mind" .)