I carefully observe trouble — not good, putting it tersely (2,5)
I believe the answer is:
in brief
'tersely' is the definition.
Both the answer and definition are adverbs. Perhaps you can see a link between them that I can't see?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for in brief that I've seen before include "To cut a long story short" , "Where barrister finds the facts" , "Summing it all up, with just a few words" , "shortly" , "To summarise or put succinctly" .)