Sloppy way to describe a broken promise (6)

I believe the answer is:

unkept

I'm a little stuck... Click here to teach me more about this clue!

'sloppy' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

I've seen this clue in The Irish Times.

Want a hint initially instead of a full solution? Install my app