Ignore! That's a smart girl (5)
I believe the answer is:
cutie
'a smart girl' is the definition.
(I've seen this before)
'ignore that's' is the wordplay.
'ignore' becomes 'cut' ('cut' can be a synonym of 'ignore').
'that's' becomes 'ie'.
'cut'+'ie'='CUTIE'
(Other definitions for cutie that I've seen before include "Attractive young woman (slang)" , "something pleasing" , "Appealing young lady" , "Attractive person or animal" , "considered by most to be attractive" .)