Prig (5)
I believe the answer is:
prude
'prig' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for prude that I've seen before include "Easily shocked person" , "Mrs Grundy type" , "One easily shocked by impropriety" , "One is excessively concerned with propriety and decorum" , "Puritan, prig" .)