Thought this setter had élan? On occasion! (4)
I believe the answer is:
idea
'thought' is the definition.
('idea' can be a synonym of 'thought')
'this setter had elan? on occasion' is the wordplay.
'this setter' becomes 'i' (the person that set the crossword).
'had' becomes 'd' (I've seen this in other clues).
'on occasion' indicates alternate letters.
The alternating letters of 'elan' are 'ea'.
'i'+'d'+'ea'='IDEA'
(Other definitions for idea that I've seen before include "Brainwave" , "Content of cognition" , "Light bulb above head" , "Notion, concept" , "Creative spark" .)