Perfect regularity of request for dessert (5-3,5)
I believe the answer is:
apple-pie order
'perfect regularity' is the definition.
(apple-pie order means perfect order or regularity)
'request for dessert' is the wordplay.
'request' becomes 'order' (an order is a request from someone in authority).
'for' says to put letters next to each other (I've seen this in other clues).
'dessert' becomes 'applepie' (I have seen 'Common dessert ' mean 'apple-pie' so perhaps 'dessert' could also mean 'apple-pie').
'order' put after 'applepie' is 'APPLE-PIE ORDER'.
'of' acts as a link.
(Other definitions for apple-pie order that I've seen before include "Perfect condition" , "Extremely neat and tidy" , "Perfect arrangement" , "Perfect state of neatness" , "Perfectly tidy" .)