Naughty or extremely angelic, kept in line (4)
I believe the answer is:
racy
'naughty' is the definition.
('racy' can be a synonym of 'naughty')
'extremely angelic kept in line' is the wordplay.
'extremely' means to remove the middle letters.
'kept in' is an insertion indicator.
'line' becomes 'ry' (abbreviation for railway).
'angelic' with its middle removed is 'ac'.
'ac' put within 'ry' is 'RACY'.
'or' acts as a link.
(Other definitions for racy that I've seen before include "rather blue" , "Sexually suggestive" , "Spicy" , "Lively" , "bawdy quality" .)