Clear about extremely nasty theft (7)
I believe the answer is:
larceny
'theft' is the definition.
('larceny' can be a synonym of 'theft')
'clear about extremely nasty' is the wordplay.
'about' indicates anagramming the letters.
'extremely' means to remove the middle letters.
'nasty' with its middle taken out is 'ny'.
'clear' is an anagram of 'larce'.
'larce'+'ny'='LARCENY'
(Other definitions for larceny that I've seen before include "Criminal activity" , "taking offence" , "Theft of personal property" , "crime" , "Theft of personal goods" .)