Dressings are not for an impossibly virtuous person (7,5)
I believe the answer is:
plaster saint
'an impossibly virtuous person' is the definition.
'plaster saint' can be an answer for 'person' (plaster saint is a kind of person). I am unsure of the 'an impossibly virtuous' bit.
'dressings are not' is the wordplay.
'dressings' becomes 'plasters' (I am not sure about this - if you are sure you should believe this answer much more).
'are not' becomes 'ain't' (non-standard version of 'isn't' or 'aren't').
'plasters'+'aint'='PLASTER SAINT'
'for' is the link.
Can you help me to learn more?
(Other definitions for plaster saint that I've seen before include "Hypocritical goody-goody" , "One not as good as they would seem" .)