Excessively casual (6)
I believe the answer is:
sloppy
'excessively casual' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sloppy that I've seen before include "Careless; watery" , "'Careless, shoddy (6)'" , "Casual; sentimental" , "Romantic" , "Slipshod" .)