Carelees, casually dressed, not disheartened? (9)
I believe the answer is:
negligent
'carelees casually' is the definition.
I can't tell whether this definition defines the answer.
'dressed not disheartened?' is the wordplay.
'dressed' becomes 'neglige' (I can't justify this - if you can you should believe this answer much more).
'disheartened?' means to remove the middle letters (the middle or heart is removed).
'not' with its middle removed is 'nt'.
'neglige'+'nt'='NEGLIGENT'
Can you help me to learn more?
(Other definitions for negligent that I've seen before include "Of the study of insects" , "Careless of duties" , "slack" , "sloppy" , "Not minding" .)