Puritanical prude's opening dress up (5)
I believe the answer is:
primp
'dress up' is the definition.
(primping is a kind of dressing up)
'puritanical prude's opening' is the wordplay.
'puritanical' becomes 'prim' (synonyms).
'opening' says to take the initial letters.
The initial letter of 'prudes' is 'p'.
'prim'+'p'='PRIMP'
(Other definitions for primp that I've seen before include "Groom oneself elaborately" , "brush?" , "Make small adjustments to one's appearance" , "Tidy" , "Make minor adjustments to one's appearance" .)