Overly formal (4)
I believe the answer is:
prim
'overly formal' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for prim that I've seen before include "Demure" , "Prudish, proper" , "Stiffly formal and proper" , "Prudish, formal" , "Formally prudish" .)