Formally prudish (4)
I believe the answer is:
prim
'formally prudish' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for prim that I've seen before include "Old-maidish" , "Straitlaced and proper" , "Straight-laced" , "Affectedly dainty or refined" , "Schoolmarmish" .)