Concerning attitude, nearly completely calm (9)
I believe the answer is:
reposeful
'calm' is the definition.
(thesaurus)
'concerning attitude nearly completely' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'attitude' becomes 'pose' (attitude can mean a pose or stance).
'nearly' means to remove the last letter.
'completely' becomes 'full' (full could historically mean fully).
'full' with its last letter removed is 'ful'.
're'+'pose'+'ful'='REPOSEFUL'
(Other definitions for reposeful that I've seen before include "Laid-back" , "serenity" , "still" , "with calming effect" .)