Proper, before start of party, to preen (5)
I believe the answer is:
primp
'to preen' is the definition.
(synonyms)
'proper before start of party' is the wordplay.
'proper' becomes 'prim' (prim and proper).
'before' means one lot of letters go next to another.
'start of' says to take the initial letters.
The first letter of 'party' is 'p'.
'prim'+'p'='PRIMP'
(Other definitions for primp that I've seen before include "Groom oneself elaborately" , "Make fussy adjustments to one's appearance" , "Tidy" , "Tart up" , "brush?" .)