Stiffly formal (4)
I believe the answer is:
prim
'stiffly formal' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for prim that I've seen before include "Formally prudish" , "Proper (type of rose?)" , "Old-maidish" , "Demure" , "Prudishly disapproving" .)