Excessively virtuous person (5,3,5)
I believe the answer is:
goody two shoes
'excessively virtuous person' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for goody two shoes that I've seen before include "Excessively virtuous person" , "Dot's gooey show? (anag)" , "Smugly virtuous type" , "Prim and proper virtuous type" , "nursery tale - with 1a" .)