Crossword Genius

Careless and extremely restive girl (6)

Ross

I believe the answer is:

'careless' is the definition.
(I've seen this before)

'extremely restive girl' is the wordplay.
'extremely' suggests removing the centre.
'girl' becomes 'miss' (synonyms).
'restive' with its centre taken out is 're'.
're'+'miss'='REMISS'

'and' is the link.

(Other definitions for remiss that I've seen before include "Not taking care", "Negligent, lax", "At fault", "Culpable", "Careless concerning a girl?".)