Disregard northern cold in nightie (10)
I believe the answer is:
negligence
'disregard' is the definition.
(I've seen this before)
'northern cold in nightie' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'cold' becomes 'c' (eg on taps).
'in' indicates putting letters inside.
'nightie' becomes 'negligee' (I've seen this before).
'n'+'c'='nc'
'nc' going into 'negligee' is 'NEGLIGENCE'.
(Other definitions for negligence that I've seen before include "Disregard" , "Being far from careful" , "Carelessness" , "dereliction of duty" .)