Remiss not to initially put on short nightdress (9)
I believe the answer is:
negligent
'remiss' is the definition.
('negligent' can be similar in meaning to 'remiss')
'not to initially put on short nightdress' is the wordplay.
'not' becomes 'nt' ('n't' can be a synonym of 'not').
'to initially' says to put letters next to each other.
'put on short nightdress' becomes 'neglige' (I can't explain this - if you can you should believe this answer much more).
'nt' put after 'neglige' is 'NEGLIGENT'.
(Other definitions for negligent that I've seen before include "Lax" , "slack" , "sloppy" , "Not minding" , "Failing to take due care" .)