Thoughtless niggle managed to conceal onset of depression (9)
I believe the answer is:
negligent
'thoughtless' is the definition.
(thesaurus)
'niggle managed to conceal onset of depression' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'niggle' is 'neglig' which is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for negligent that I've seen before include "Careless, remiss" , "sloppy" , "Careless of duties" , "Not minding" , "Not taking due care" .)