Understood to have become popular (6,2)
I believe the answer is:
caught on
'understood' is the definition.
The answer and definition can be both related to thinking as well as being past participle verbs.
Perhaps there's a link between them I don't understand?
'to have become popular' is the wordplay.
I cannot quite see how this works, but
'to' is present in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for caught on that I've seen is " Comprehended - became fashionable".)