Great thought to change sides (10)
I believe the answer is:
celebrated
'great' is the definition.
(thesaurus)
'thought to change sides' is the wordplay.
I cannot really see how this works, but
'change sides' could be 'rat' (I've seen this before) and 'rat' is found within the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for celebrated that I've seen before include "Observed (festival); famous" , "Did something enjoyable (to mark an event)" , "Notable" , "Eminent" , "Cheered" .)