Straight-laced (4)
I believe the answer is:
prim
'straight-laced' is the definition.
(synonyms)
This is all the clue.
(Other definitions for prim that I've seen before include "Straight-laced" , "Straitlaced and proper" , "Affectedly formal" , "Proper (type of rose?)" , "Strait-laced - prudish" .)