In test, put a stop to breaks (6)
I believe the answer is:
trendy
'in' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?
'test put a stop to breaks' is the wordplay.
'test' becomes 'try' (synonyms).
'put a stop' becomes 'end' (I can't explain this - if you can you should believe this answer much more).
'to breaks' means one lot of letters goes inside another.
'try' going around 'end' is 'TRENDY'.
Can you help me to learn more?
(Other definitions for trendy that I've seen before include "Informally, following the fashions" , "following latest convention" , "'Hip, fashionable (6)'" , "Smart" , "In fashion, of popular style" .)