Overly proper (4)
I believe the answer is:
prim
'overly proper' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for prim that I've seen before include "Prudish, proper" , "Straitlaced and proper" , "Old-maidish" , "Prudishly disapproving" , "Demure" .)