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