Lying in coma, due out before 1st of September (10)
I believe the answer is:
mendacious
'lying' is the definition.
(mendacious means dishonest)
'in coma due out before 1st of september' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'before' is a charade indicator (letters next to each other).
'sst of september' becomes 's' (1st letter of 'september').
'in'+'coma'+'due'='incomadue'
'incomadue' with letters rearranged gives 'mendaciou'.
'mendaciou'+'s'='MENDACIOUS'
(Other definitions for mendacious that I've seen before include "falsifying accounts?" , "Given to lying" , "Untruthful" .)