Nancy's very cross and that's wrong (8)
I believe the answer is:
trespass
'wrong' is the definition.
(I've seen this before)
'nancy's very cross' is the wordplay.
'nancy's very' becomes 'tres' ('tres' means 'very' in Nancy, a French city).
'cross' becomes 'pass'.
'tres'+'pass'='TRESPASS'
'and that's' acts as a link.
(Other definitions for trespass that I've seen before include "Sin (archaic)" , "Transgress" , "Enter land illegally" , "Commit a sin -- spa's rest (anag)" , "Illegal entry" .)